When I'm writing a contract I usually put this in the milestones I'm paid by. E.g. one week for feature 1 delivery, next week for client requested adjustments t

When I'm writing a contract I usually put this in the milestones I'm paid by. E.g. one week for feature 1 delivery, next week for client requested adjustments to it, next week for feature 2 delivery, week after for client requested adjustments, etc.. Generally I only pick something I could finish in half a week for the 1 week milestone too, so there is plenty of time if it turns out tougher than I thought. This is also a good way to communicate that you won't tweak a feature every week for months after without adding another milestone (and more pay) to compensate, something every client always asks for in the end, because you don't really learn anything until a feature is put in front of an end user.