To get back home I have a hourly train that leaves FRA long distance train station at XX:52. If I land at Terminal 1 A or B gates, my cut-off is 40 minutes. If I plan to land before XX:12 I book the next one and if not I book one hour later. Bare in mind I am often HBO and I also know my way around FRA.
If I land at T2 that would be one hour.