My policy is at least 24 hours out and that they must be fever-free with no medication to suppress fever. I also like to factor in the child's energy, too. Bringing a lethargic kid with no fever to daycare isn't doing anyone any favors, kiddo needs to be home. This, by the way, is also my criteria for keeping my son home--if he seems to be dragging, I don't have him go to school, fever or no. I would remind the mother what your sick care policy is (do you have one in writing? It would be helpful) and let her know that you'd love to have the little one back when she's feeling better.
I think this policy also eliminates the chance that the parent might use Tylenol to suppress a fever and then drop the child off, knowing that we were going to take their temp. (This happened to me several times at the daycares I worked at.) Their kids would be so sick, and they would bring them in because the parents needed to work, even if it was only to get a few hours in before we caught on. But we always did. It was pretty embarrassing for everyone. Your policy is a good one!