Layover in Frankfurt - enough time?

As long as the first flight arrives on time you will make it. Otherwise if you don't the United is obligated to put you on the next flight with space available and in a hotel at their expense if there is no flight with space available.
United has two to San Francisco daily:

1:35 PM –
4:05 PM
UnitedLufthansa
11 hr 30 min
FRA–SFO
Nonstop

5:30 PM –
8:00 PM
UnitedLufthansa
11 hr 30 min
FRA–SFO
Nonstop

I would never book a flight with only a 70 min layover in Frankfurt, especially with the second flight being the long flight home. There may be another flight that same day but will you be able to get on it? And if you do, will you be in a middle seat or since there are two of two of you, separate seats for that 12 hour flight home?

Thank you, everyone, for taking the time to reply. Very informative and reassuring, and helped us to know what to expect at the airport. We're going to keep our itinerary as-is. At least if we miss a connection and are delayed, it's on the trip home and not to Italy. Cheers!

We just returned from Lyon France through Frankfort with a 1 hour connection. I had questioned the short interval and was assured it would be enough time. Some on this forum have stated that if they put you on the bus you will get on the plane. Our flight from Lyon was on time and we progressed through the airport. The bus transfers were long and we barely made it to the gate within the allocated time. Meanwhile they had given our seats to stand by people (because we were “late”) and though we did get on the plane we did not have the seats we had so carefully chosen. That flight was an hour delay in take off so they could load bags ( mine did not make it on the flight!). All’s well that ends well but would not go along with such a short connection again. I hope it all works out for you with no complications!

I didn’t understand it either. We were checked through to Grand Rapids MI through Chicago, had boarding passes and seats. When they scanned our pass in FRA it came up as “passenger not found”. Had to see a desk agent, who called several names to board before trying to figure out what was going on with us. I assumed they were stand by customers. In any case we did not get the seats we reserved but did get on the plane. For what’s it’s worth we would have missed our connecting flight out of Chicago (3 hour layover) had that flight not already been canceled and rebooked for later. In the future I’ll pay more attention to clearing everything once I land in the states if I still have a connecting flight to get “home”. This forum has been invaluable to me in trying to negotiate our new normal. Thank you to all who contribute!