I did a search for this about a week ago and finally found a thread by searching "housekeeping," but I apologize as I didn't subscribe or bookmark the thread.
Basically it boiled down to that most people tip $1 per person per day with $1 extra per day. For example, if you have four people in your room you would then tip $5 per day. It was advisable to tip per day in case you had differrent mousekeepers on different days.