If I fly BA from LHR to FRA (land in terminal 2), then AA from FRA to DFW (depart terminal 1) - how much time do I need between flights? Carry-on baggage only. One ticket for all flights.
A few years ago, we flew into FRA from Munich, deplaned at Concourse A (T1) and went through the tunnel to Concouse B, still T1, to fly to Denver. We had about an hour and it took us the whole time. We had to go through security 3 times. If you use the sky line to get between terminal, it shouldn't be a whole lot worse. We also had only carry-on bags.
It might depend on what concourse you are going to in Terminal 1. There is a skyline station on top of Concourse A and one near Concourse B. For Concourse C, you might be better off to walk from Terminal 2.
See map.
There is a lot of construction going on at the airport. The people mover between T1 and T2 is currently being extended to other concourses and therefore doesn't run as frequently as it would normally do. There are bus services between the terminals as well. AA is in T1 C which is a bit of a walk from where you'd get into T1 coming from T2. Also flights rarely leave Heathrow on time so you should allow at least (!) 2 hours for your transfer in Frankfurt.