Following suggestions from other threads, we now tip on a daily basis, to ensure the individual doing that day's work gets the tip. Otherwise someone may luck out on the last day when they have been the one not doing the work during your whole stay. Anywhere between $2 to $5 per night, depending on how messy the room is.
Gotta add, I never tipped housekeeping before reading these boards. You learn something new all the time!