I heard somewhere $1 per person, a little more if there is a big mess. We are a family of 3 and always leave $4 or $5. I put it in a plain envelope and write "mousekeeping, thank you!" on it. If I forget to bring envelopes I just fold it into a piece of paper and write "Mousekeeping" and leave it propped against the sink.
make sure you tip every night, not just at the end. They change mousekeepers midweek sometimes.