Hi there. i started using the robert beck zapper a month ago. i use it for 2 hours daily and its brilliant. My partner has just bought himself a Don croft zapper, i think they are all pretty much the same. The one he uses is portable so can be worn for as long as he likes. Mine is a bigger box with 2 wires and crocodile clips that have to be attached onto biotabs so its not feasible to wear for long periods because you have to sit still while wearing it.
The bottom line is if its portable it can be worn for as long as you like. some people use them overnight. just stick to a routine, use it every day at the same time then at least you are zappping often enough to keep it working at its best.