When doing the initial appointment, and they have a kitchen/canteen area, I always ask about who is responsible for collecting, cleaning, and puting away any cups and pots. If they have a dishwasher then it is easy but I have a few that do not. I always add it into the price. But as you have said it is hard to tell how many there will be.
The process I use does not usually require much more time if done correctly. All cups are collected during the regular work, such as vacuuming, emptying bins or wiping desks (which ever you are doing first). They are then gathered at the sink, again as someone else stated I do them right away but with out bleach. The reason is to save time later. I let them stand and dry during the rest of the shift and then put them away, it saves the time of drying them with a cloth. So if you collect them while doing the intial tasks that require you be at each desk anyway, wash them and let them dry on there own, you really only have the time of washing each one and putting them away unless you agree that they will put them away the next day and if there were quite a few I might go down that route but right now I am able to accomplish it all without much increase in man hours but the client is usually quite happy to have a little put on top for that.
Hope it helps
Gibert