When training for trad, I used to let lads do downstairs for at least a month, many twice as long. When I could see they were doing them right, then I would start to let them do some upstairs work, but half a day at a time.
Their muscles need developing to be able to work all day, so easing them into it seemed to work.
I used to find it took 3 months to get them to the point where I would trust them to clean houses on their own. That might sound like a long time, but it meant that when they started to do the other side of the street, I knew they were looking after my customers.
Wfp totally different, start them on the upstairs straight away, show them the technique and let them get on with it. But I would do any first cleans, for the first couple of weeks.