10 on-20 off-10 on-20 off-10 on- off Is good and repeating 3 times daily for a single frequency zapper is about as much as you can expect.
Sadly, but only partly as Jaguar57 claims, the single frequency zapper is very limited. Far better are multi-frequency zappers that are proven to be highly effective in killing many microbes.
Even better are the ones that offer highly accurate Rife frequencies such as ParaZapper.
Multi-frequency zappers are not used 7 on or 10 on then off, but are to be used continuously for an hour or even longer depending on the number of frequencies.