I only tipped at Christmas but the maid was the owner, not a maid working for a business.
My friend who has a maid that comes thru a service only tips $5... which I think is kind of low.
I really think it depends if she's the owner or if it's a service. If it's a service you want to hand cash to the actual maid out of sight of the boss.
Maids thru a service are paid very low. Usually by the day and not by the house, at least that's how it was for mine who now owns her own service. She said under a boss she got $50/day and she might clean 2 or 3 houses. Usually one was her primary house to clean, but the boss would pick her up and take her to the other houses to help finish off. Doesn't seem fair.