You can set evolution to loop but the fractal is not based on a grid so there are no repeating edges to match up. Your best option is to create a comp that is at least twice as long as the comp you need, add a layer with fractal noise that is animated the way you like, then duplicate the layer, slide the top copy down the timeline, set the blend mode to difference, and look for a place where the two copies of fractal noise start to cancel themselves out and give you a black comp panel. Set the blend mode back to normal, add a comp marker, then make a cut between layers at this point and see how well the cut is hidden. When it's acceptable, set an out point for the comp using the work area and trim the comp. You'll just have to get as close as you can and live with the glitch.
Another option might be to add fractal noise to a very wide solid, animate position instead of offset and find the overlap and do the same thing.
Personally, I never mess with looping fractal noise. If I can't get the noise to last long enough I just duplicate the layers and do a transition between them.