FRA is a monster airport. I've made it from one end to the other with a customs, security checkpoint and being lost in less than an hour. That was coming from the US and making a connection to Berlin.

I would be more concerned with the connection time on the way back to the US. The security checkpoints held us up for almost an hour. Also once you are through the checkpoint there is "NO" place to get anything to drink. We found it the same on a recent trip back from CGN also. after security to the US you get nothing!

Good luck.