If I am staying by myself for just a night and I don’t make a big mess, $2. If I’m there multiple days and kind of explode all over the room (as I am wont to do), $3 a night. With my family, $5 a night (3 of us). If we are in a major city or a resort (Disney) then I will usually leave something bigger on the last night, up to $10.
When I was at Disney with my mom, brother, and son, in a suite, I was tipping $10 a day plus $20 at the end because they would have had to clean out the fridge and kitchen area, too. My mom is a notorious cheapskate so she didn’t get it, but man, no one gets paid enough to clean up hotel rooms. That is a fact.
In your group’s situation if you had rented out the whole place for 2 nights I’d probably do $10 per room plus $20 per common area.