Join Date: Dec 2006
Location: Pacific Northwest
Programs: UA Gold 1MM, AS Plat, AA EP, Bonvoy Plat, Hilton Dia, Hyatt Glob, IHG Plat, ...
Posts: 21,296
LH is operating FCO-FRA, right? Then you’d stay within T1.
Single ticket?
Do you have a passport eligible for the automatic immigration gates? That would likely speed up the exit passport control.
For me 90 mins would probably be okay if I didn’t have better option (earlier flight from FCO), and also depending on how much of a disaster it would be to miss the connecting UA flight.