I just imported about 20 minutes of backing tracks, yet the Free Loop Minutes in the about page (hold down genre) only went down about 5 minutes.
Don't worry about the loop time remaining not going down as quickly as you might think.
Loop minutes are calculated based on 32 bit loop storage with 2 stereo tracks per loop track (so 6 stereo pairs). The amount of memory for a loop slot/track is significantly higher per-minute than backing track storage. Backing tracks are only 1 stereo track.
Therefore, you will be able to get more than 3 times the suggested loop time if using backing tracks.