TLT average duration is 17yrs bond,

You can roughly refer to 20y bond yield to estimate. Right now, 20Y Bond has already come down from 5% in the beginning of the year to 4.2%.

Fed rate only controls overnight borrowing rate, so, it is not a direct measurement, but with Fed starting to cut rate aggressively, i estimate 20Y bond will drop to 3%-3.5% range within 12 month.

Averaging this up, we can use 3.2% as a base to estimate which means 20y bond yield will go down another full percentage point. TLT price is likely to appreciate ~15% +/- from here in 12 month.