Time needed for PRG-FRA-JFK

You are staying inside Terminal 1. I know I can walk from one end of Terminal 1 to the other at a very casual pace in 35 minutes (and not so casual a little less than 20, I did it before and I didn't even kill anyone en route :P frankly, it was more of a speedwalk than a mad dash). The official MCT (Minimum Connection Time) is just 45 minutes and you have double that. There will be an exit border check but it's fast, barely more than a formality. 1:30 is plenty. (At Frankfurt! We are not talking of Heathrow. There... uh, you can try but it'd be a very, very close shave.) Nonetheless, if you can, try to get a front aisle seat on the Prague flight so you don't lose a lot of time deplaning.