Could there be a leak in the air conditioning in my car?

I've had this problem for a while, but finally decided to do something about it. When I turn on the cold air in my car, it works for maybe 25-45 minutes staying cold, but after a while, it would start blowing warm air. This happened last weekend and I figured maybe it was because my dashboard was hot. But I tried it today and it was a little chilly out, but my air was icy cold. I don't know what's going on. Could there be a leak somewhere? Or could this be a reminder to put more freon in my car? Help!!!