What is an acceptable tip?

My husband and I are staying in a hotel (for his job) for two months now, and will be here for quite a while longer...anyways, what should we be tipping the housekeepers? We don't always get the same one, some aren't very good, others are...I want to tip the good ones, but I don't know who cleans when. I don't want the good ones thinking we're rude.

buy low, and sell high

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tip based on their performence
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leave a dollar or two,,if you like one that is good you can give her more,,,,as they will go the extra to help you on what you need,,
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$5 and other 2%