I know this is a hot topic for some but I've heard the standard is $1.00 per person per day. For us this meant $5.00 per day. One night, my was son was sick on the bedspread. Though I cleaned it up, I wrapped the bedspead in a ball and called housekeeping to explain and ask for a replacemnt. I left $10 that day. I usually put it on a clear space on a table and put a note with it such as Housekeeping-Thanks!
At a hotel where there was nightly turndown service, I just left $2 or $3 those nights (in addition to day tip) because it was opening the beds and emptying trash barrels.