Enemas start to work immediately. For maximum benefit they need to be done frequently (e.g. every day).
The only thing you have to worry about is that you don't become dependent on enemas for bowel-movements - which is a theoretical risk, but has never bothered me. What I do is wait until after my normal morning bowel movements before doing my enema at around midday. That way my bowels get to work normally every day.
Of course there will always be *some* stools in there, but you want it to be as little as possible.