I normally leave $5 per day. On our last trip we stayed at Paris. When we checked into our room, I noticed food crumbs on the floor. I also noticed hair in the shower. I left $5 on the table the next morning. When we returned to the room, the housekeeper took the $5, but the crumbs were still there, as was the hair in the shower. Apparantly all she did was empty the garbage cans and make the bed.
The next day I left no tip. But it didn't matter because housekeeping never came. I know I could've called and complained, but I don't really care whether they come or not. As long as I have enough clean towels, soap, toilet paper, etc, I'm okay.
I've been to Vegas many times and this was the worst housekeeping I've ever seen. The third day ( I left no tip this day either) I came up to the room in the afternoon to take a shower, change clothes, and get more money from the room safe. Two maids were standing by their carts chatting in Spanish across the hall from each other. Thirty minutes later when I left the room, they were still chatting back and forth. Now I knew why they weren't getting the rooms clean. Needless to say, other than the first day, I didn't tip these useless maids for the rest of the trip.