This is a difficult question to answer because every employee is so different. And it also depends on if you’re talking about a functional knowledge or really knowing the whole system. If you look at some of the POS systems websites they offer trainings that range from 30mins to 2+hours (over several visits). Along with this some people function best from reading the manual (so the learning time depends how thick the manual is). But really what it comes down to is your employees. Some more tech savvy workers may pick up the POS system with in that 5-30 min introduction to the system and never have to ask another question about it (Chances are they may not know it but can figure things out for themselves). And some employees will need more hand holding and may never learn the entire system and may have to always ask about infrequently used functions. What it comes down to is a bell curve of learning. I would say that within a week of using the system most people should be able to operate the basic daily functions of the system and make minimal errors. However, there are the two tails of the bell curve, those who will get it the first minute and those who may always have trouble with it (both of these being rare circumstances). Best of luck and remember to support all your employees and their different learning needs and speeds.