I don't know if it's been said yet- but here's what I personally do, because I had alot of these when I first started doing commisssions.

Sketches are done within a couple days of payment, the customers agrees or edits slightly. If large edits or complete revisions are required, they must pay the price of a sketch commission (because thats what they're essentially making me do). After the sketch is approved by the commissioner, there will be no guaranteed changes to it. They can ask, and I can say no if I want. Point is- I shouldn't have to redo sketches or change them dramatically if I get a decent description in the beginning.