Why doesn't my timeline duration change when I put the same time lapse (image sequence) into comps with different frame rates?
Why should it? Time is time and frames are frames. A 3 second clip at 25 fps will still be 3 seconds in a 30 fps comp. If you want to change that, you have to change the source framerate in teh footage interpretation.
Mylenium is correct.
This might help you to understand better:
Retrieving data ...