As a villa owner on St. Barth, here's my perspective:
The housekeeper is there to basically clean the house and linens, not be a personal servant. That's why I use the term "housekeeper" instead of "maid." It's one thing to leave a few dishes in the sink, but another to expect her to clean up the entire kitchen, including all the pots, pans, and dishes after a large dinner party.
Most housekeepers on the island are happy to do a few favors for the client if they are nice to her. Many will do extra work, such as laundry or shopping, for the client in the hopes of getting a generous tip. It is important for renters to realize that the tips are an important part of a housekeeper's annual income.
I think that JEK's suggestion of 10 euros per bedroom per day is reasonable, or more if the housekeeper does a lot of extra work.
~ Jean d'Arme ~