Time needed to change planes at FRA

it will help to know if your flights are in different terminals, and if you have any checked luggage that will need to be picked up, or if it is checked through to final destination. Assuming the trip was booked on one ticket?

Remember you have to do the immigration control to the EU. This is faster than in the US, but can, if many travelers come, take some time to complete.

What is your final destination?

That only gives you an hour and forty five minutes if everything goes well. And how often does that happen anymore? Several times we've had flights delayed that long on take offs due to mechanical issues. Then there are problems with weather and many flights arriving at the same time. I'd play it safe and go for the 2:20 flight. If you wind up with time on your hands you can always get a bite to eat.

A United flight undoubtedly arrives at Terminal 1. KLM uses Terminal 2, so you will have a considerable distance between gates. Play it safe, take the later flight. Of course, if you take the earlier KLM flight and it's a single booking, and if you miss the earlier flight because of a late arrival, they will probably put you on the later flight, anyway.

Final stop is AMS, we did not intend to book with one ticket but will look into that. Thanks everyone for your info, we will play it safe and book the 2:20 flight.