Question about balance on pre paid plan

I have prepaid $.25/min plan. Does the money on the balance expire every month? whether I use or not? I do not expect to use it much, just emergencies, loaded it with the minimum and thought it would last for several months, but now looking at the website I am wondering if the balance is going to be deducted after 30 days no matter if we make no calls at all. Is that right?