If you zap for 3-1/2 minutes on a single frequency, very few microbes will be killed.
Even zapping for 5 to 6 minutes does not kill very many microbes. That is why some zapper sellers recommend reduced zapping time to avoid die-off.
Zapping 7 minutes will kill many but not all microbes. For that reason, there are almost as many microbes the following day. It is also why many people suffer die-off day after day after day.
Zapping for 10 minutes will kill more microbes of any given type than 7 minutes does and it also reduces die-off from the following days zapping.
However, even zapping for 1 hour continuous with a single frequency will not kill all kinds of microbes.