When you want out, your entire
deal is to let the client know, and give parameters for closing out the working
relationship. Unless the relationship is toxic (in which case, you want to let
the client know you're out TODAY and will mail back anything of hers you have,
and send a final invoice), it can go something like this:
"Joan, I've decided to take my business in a different direction and we need to wrap up our working relationship. The last day I'll be available will be _____ " If you need help finding someone else I can make some suggestions for how you might do that, and if you find someone soonish, I'll be on hand to help with the transition. Otherwise, I'll make sure you have all you need to help a new person get up to speed."
Fill in that blank with a reasonable date--no less than two weeks away and no more than a month.