I have done it based on availability. I request a minimum of 24 hours notice, and I reserve the right to refuse based on space (or me just not wanting to, not that I would tell them that, lol).
I am not a huge fan of it. The little girl would spend most of the morning crying for mamma. The blessing was that she was a GREAT sleeper. Honestly, if she had nap issues, I would have stopped. And at first, dad would just sort of show up whenever he wanted to. But I explained that I needed to know when to expect them, so that we weren't waiting for the doorbell all morning. I am not okay with our schedule being thrown all out of whack, just because your appointment isn't until one.
They fill out all the same paperwork, even if it's a one day a month thing. They pay daily, at the end of the day. No pay, they won't be welcome back.