I like to leave around $3.00 a day: always with a note thanking the housekeeper. These folks have one of the worst jobs in the hotel with low pay. Once in a while I get a note thanking me. Often when I encounter them they will ask what else I would like (usually it is something as easy as only regular coffee and plenty of creamer). They are genuinely greatful for the tip and I am happy that I could leave a few bucks and help make their day better.