That is so wonderful!
I remember the first time Cooper rang his bell. It could possibly have been the cutest thing I've ever seen. It took him a couple of weeks. I trained Max to ring the bell also and it took him a little longer. If I don't respond to Cooper he'll ring it really hard and then growl. Max is so uncoordinated that he'll ring it about 3x every time and then it flips over and he just starts crying.
Make sure you give treats ONLY when Maggie rings and then potties. You may start going through a very frustrating time where she realizes that the bell = outside (not just potty). Be patient with her. It may take quite a while longer before it's perfected.