I run 3-4 times a week for about 25 minutes. How long should someone run for to optimize fat burn? I've heard some say 20, 25, 30 mins. is longer better ? My goal is not to become a marathon runner. Im just trying to change body composition. Thx.

The longer you run (at the same intensity) the more fat your gonna burn. 25-30 minutes sounds like a good place to start.

If your trying to optimize fat burning (long term) you should try running some intervals. Too much steady cardio has a negative effect on your body composition. I'd do some weights (or some type of resistance training), run intervals and watch your diet like crazy.

How are you eating right now? That's the main thing if your trying to lose weight.

check this link for good info on interval training: