When do u tell someone no?

I get what you are saying, you are doing her a favor, yes she is paying you, but still you are doing it as a friend and she is taking advantage of you. Does she even call before she comes to ask if you are available? Maybe you should tell her that you aren't able to help out as much as you could before. Tell her you're sorry but that you aren't at her beck-and-call. You can still be there for her to help her, but lay down a few rules so that you don't get walked all over.