I give them dirt every 3 to 4 days. That's often enough to keep the right bugs in their system so they can gain immunity.
How much for grit. It doesn't take a lot. How long it lasts depends on what rocks you have in your soil. Some rocks are hard and last. Some are soft and get ground up...
I'll copy something I wrote on medicated feed which should answer a lot of questions on how it works. It is a long article but maybe you can make a more informed choice.
First you need to know what the "medicated" is in the medicated feed. It should be on the label. Usually it is Amprolium...