First of all, United does not have a flight from Frankfurt to Atlanta - you probably booked a codeshare that is operated by Lufthansa.
Assuming that you booked both legs separately (i.e., they are on separate tickets), you roughly need about 2 hours in Frankfurt, as you will need to take the luggage from the belt and bring it to Lufthansa check-in. According to their web-page, check-in closes 40 minutes before departure, but since the flight is to the US, you should be there way earlier (my guess: 90 minutes). Getting from the gate to the belt is quite quick, as passport checks (if conducted) are quick. After the check-in, you will need to go through security again. Passport checks are typically quite quick. Having said that, the 2 hours are just a rough figure and by no means a good guideline. I would personally never book a seperate-ticket connection if the connection time is not at least 5 hours. If your incoming flight is delayed, your out-going one (from Frankfurt) is void, including the return trip. Also, you may be taken to the gate from the plane by bus, which costs extra time.
If both flights are on the same ticket, then do not worry - the travel agent or the web-site has already checked that the minimum connection time has been met and if your incoming flight is delayed, you will be rebooked free of charge.