I'm not aware of there being a maximum limit. I've never had to add more than $20 every 90 days. And I don't use it often at all. The most I've had on it at one time was a little over $100. Usually I have at least $50 on it all the time. They sometimes have a deal that if you add a certain amount with 90 days you don't have to add anything for a year unless you need to.
They've added some different plans recently that seem to be good deals if people use it a lot. I've stayed with the basic plan as long as I've had it. 25 cents a minute for the first 10 minutes of each day. Then the rest of the day it's 10 cents a minute. Texting I believe is 5 cents.