As mentioned there is no need to go via Frankfurt main station.
The ICE trains only depart from Fernbahnhof, Regionalbahnhof is only for local trains. However, the S-Bahn train (regional trains) now also run via Fernbahnhof as there is construction work until December 14, which also affects long distance trains. But the homepage of Deutsche Bahn is in so far up to date in regard to the current timetable.
Said that, I would at least consider 30 minutes from passing immigration to arriving on the "Fernbahnhof" (long distance trains). Use the
SkyLine (if running). It is signposted and departs on the upper level (level 4) of terminal 2. As seen
here , the access is pretty much in the center of the terminal 2. Yo will arrive in terminal 1 in the main terminal as well (close to access to the B gates). You need to follow the signs to Fernbahnhof, which is one level up and then walk via connecting covere pedestrian bridge to the Fernbahnhof.
You can also follow the signs to Sheraton / Marriott (if you access the hotel from the right, you need to pass the lobby) and Hilton / Hilton Garden Inn (sitting atop the station,).
If you are traveling very light, you can walk from terminal 2 to terminal 1 in 15 minutes.