A PCR machine usually takes 45 minutes to 2 hours to amplify DNA. A standard run of 25 to 35 cycles for a product of a few thousand base pairs falls inside that range, a 40-cycle qPCR run takes about an hour, and a fast polymerase with short holds can finish in roughly half an hour.
That is thermocycler time only. Checking an endpoint product on a gel usually adds another hour or two, and a diagnostic PCR test sent to a laboratory can take up to three days from swab to result, most of it spent outside the machine. The table below keeps these three readings of the question apart.
How long does a PCR run take?
A typical PCR run takes 45 minutes to 2 hours on the thermocycler. Bustin describes a 40-cycle reaction as taking about 45 minutes to an hour, depending on the protocol and instrument,[1] and the National Human Genome Research Institute notes that the whole automated cycling process can be completed in a few hours.[2]
| What is being timed | Typical time | What sets it |
|---|---|---|
| Standard endpoint PCR, 25 to 35 cycles | 45 minutes to 2 hours | Amplicon length, polymerase, cycle number |
| Fast PCR or fast qPCR protocol | About 20 to 35 minutes | Short holds, fast polymerase, cycler speed |
| qPCR, 40 cycles | About 45 to 60 minutes | Instrument and hold times |
| One-step RT-qPCR, 40 to 45 cycles | About 20 to 66 minutes of holds, plus ramping | Reverse transcription and annealing time |
| Gel check after endpoint PCR | About 1 to 2 hours | Electrophoresis and staining |
| Diagnostic PCR test result | Under 2 hours at point of care; up to 3 days through a laboratory | Transport, batching, extraction, reporting |
Machine time is the number of cycles multiplied by the length of one cycle, plus a few minutes of setup and finishing holds. Each cycle is the sum of its programmed holds and the time the block spends heating and cooling between them. Bustin points out that run speed depends on these ramp rates as well as on the holds, and on cooling speed in particular.[1]
Specialized equipment can go much further. Bustin's review describes short amplicons copied in under 15 seconds and a commercial cycler that amplifies a 100-base-pair target in just under two minutes, although such systems usually trade away some efficiency or need dedicated hardware.[1] For an ordinary block cycler, the holds set by the polymerase are where most of the time goes.
How long does each PCR step take?
Each cycle has three holds: denaturation for 5 to 30 seconds, primer annealing for 15 to 30 seconds, and extension, which scales with the length of the product. Extension is where polymerases differ most. New England Biolabs recommends 1 minute per 1,000 base pairs for Taq, compared with 15 to 30 seconds per 1,000 base pairs for the high-fidelity enzymes Q5 and Phusion.[3][4]
| Step | Taq | OneTaq, Vent, Deep Vent | Q5, Phusion |
|---|---|---|---|
| Initial denaturation | 2 min at 95 °C | 30 s at 94 °C | 30 s at 98 °C |
| Denaturation, each cycle | 15 to 30 s at 95 °C | 15 s at 94 °C | 10 s at 98 °C |
| Annealing, each cycle | 15 to 30 s | 15 to 30 s | 15 to 30 s |
| Extension, each cycle | 1 min per kb at 68 °C; 45 to 60 s below 1 kb | 1 min per kb at 68 or 72 °C | 15 to 30 s per kb at 72 °C |
| Final extension | 5 min | 5 min | 5 min |
Because extension repeats every cycle, product length quickly dominates the run. A 3-kilobase product amplified with Taq needs 3 minutes of extension per cycle, or 90 minutes over 30 cycles before any other step is counted. The same product from genomic DNA with Q5 needs 1.5 minutes per cycle, or 45 minutes. For very long targets, NEB's LongAmp Taq is run at 50 seconds per kilobase, while NEB advises against extension times longer than recommended for proofreading enzymes such as Q5.[4]
Summing the holds in NEB's typical 1-kilobase protocols gives 22.2 minutes for Q5 or Phusion over 25 cycles, 43 minutes for Vent or Deep Vent over 25 cycles, and at least 50.5 minutes for OneTaq over 30 cycles.[4] By the same arithmetic, standard Taq with a 2-minute initial denaturation and 30 cycles comes to 52 minutes. A real run is longer by the ramping time, which depends on the instrument.
A fast enzyme only saves time when the reaction works the first time. High GC content, secondary structure, or primers with mismatched melting temperatures can force longer holds, a different annealing temperature or a repeat run. Designing primers in Primer3 and checking them with an oligo analyzer before the run costs minutes; a failed run costs another hour or more.
How long do qPCR and RT-qPCR take?
A 40-cycle qPCR run commonly takes about 45 minutes to an hour. QIAGEN gives around 60 minutes for 40 cycles,[5] and Bustin measured 49 and 51 minutes for a standard protocol on two instruments, which fell to 33 and 31 minutes after lowering the denaturation temperature and shortening the holds.[1]
Quantitative PCR, or qPCR, reads fluorescence during cycling, so the result is available when the run ends. Data acquisition and an optional melt curve can add time, but qPCR avoids the gel that conventional endpoint PCR usually needs.
Reverse-transcription qPCR, written RT-qPCR, first converts RNA into complementary DNA. A current Thermo Fisher one-step protocol specifies 5 to 15 minutes of reverse transcription, 2 minutes of enzyme activation, then 40 to 45 cycles of 5 seconds of denaturation and 15 to 60 seconds of combined annealing and extension.[6] We calculated that these holds add up to between 20 and 66 minutes, depending on where in each range a protocol sits, before ramping is counted. A two-step workflow performs reverse transcription in a separate reaction and takes longer at the bench.
The abbreviations are easy to confuse. The MIQE reporting guidelines use qPCR for quantitative real-time PCR and RT-qPCR for reverse-transcription qPCR.[10]
How many PCR cycles are needed?
Most conventional PCR protocols use 25 to 35 cycles; qPCR and low-copy targets often use about 40.[11][12] Every extra cycle adds its full set of holds to the run, so cycle number is the second lever on run time after product length.
More starting template needs fewer cycles. QIAGEN's guidance ranges from 25 to 35 cycles for more than 50,000 copies of a single-copy target to 40 to 45 cycles for only 10 to 100 copies.[12]
Under perfect doubling, one starting molecule would become about 33.6 million copies after 25 cycles, 1.07 billion after 30, and 34.4 billion after 35, calculated as 2cycles. QIAGEN describes this doubling as the theoretical case.[12] Real reactions copy fewer than every template in each cycle, which is why the MIQE guidelines ask qPCR studies to report amplification efficiency.[10]
Adding cycles is not a free way to rescue a weak assay. Lorenz notes that going beyond 35 cycles often enriches unwanted secondary products.[11] Template quantity, primer specificity, polymerase choice and the detection method should set the cycle count. For typical input amounts, see how much DNA is needed.
How long until you have a result?
The cycler is often not the slowest step. After endpoint PCR, the product is usually checked on an agarose gel. In a published teaching protocol, Lee and colleagues separated fragments at 100 volts for 1 hour, and post-staining with ethidium bromide takes another 15 to 30 minutes plus an equal time to destain.[7] Adding the run itself, we estimate that a research PCR takes about 2 to 4 hours from setup to a photographed band.
Diagnostic PCR tests stretch further. The CDC says that laboratory-based nucleic acid tests, which include PCR, may take up to 3 days to return results after a sample is collected and sent away.[8] Most of that time goes to transport, sample registration, nucleic-acid extraction, waiting for a full plate and reporting, not to cycling. Point-of-care molecular tests remove the trip: a Cochrane review defined them as tests needing minimal equipment and giving results within two hours of sample collection.[9]
Put together, PCR time nests in layers: seconds for each step, under a minute to a few minutes for each cycle, about an hour for a run, and hours to days for a reported result. Speeding up one layer helps only when it is the bottleneck. A faster polymerase can save half an hour of cycling, but it cannot shorten the courier's route to the laboratory.
