Finally took the time too find the loop time for a generator. Caustics loops at about 25 seconds and 4 frames when speed is set to 1. It's a weird number. I haven't tested other generators. Wish they would let you set a loop function on generators. OH well. Hope this helps peeps! I meant for this to be posted in Motion forum, Oh well.
Apple Motion has a specific tool for looping generators, but I don't think a lot of people really know about it.
It's in the Markers.
Make a marker,
Edit the marker and change the type to Project loop end.
This way if you make a project that is 8 seconds and place a loop marker there, inside final cut it will loop that 8 seconds.
The trick is to make the generator seamless at that point.