Primer extension typically occurs at a temperature ranging from 68-72°C. Repeat steps 2-4 25-30 times.Final Extension for 5 minutes at 72°C: A final extension to fill-in any protruding ends of the newly synthesized strands. In general, 1 min per kb is very practical as duration of extension time. There could be a lot of other issues if you are not getting a band. The recommended extension temperature is 72°C. Repeat steps 2-4 25-30 times. Final Extension for 5 minutes at 72°C: A final extension to fill-in any protruding ends of the newly synthesized strands. PCR products up to 500 bp. It's the latter the primers continue to anneal during the extension step because it is a 2step PCR (98°C 15sec, 65°C 5 mins).