How Often to Repack Trailer Wheel Bearings?

I repack when I change tires.Same thing for Single Engine Airplanes. Works for me and I use Timken bearings on everything. I have never had a Timken failure and having had a front bearing on a Chevy 2500 even run Timkens on them. As for grease. almost all grease meets NLGI 2 requirements it will work. NLGI goes from 000 to 6, 6 being thickest. We have farmed for years and use 2 for hi speed swathers, mowers and balers with good results. don't get hung up on grease. Wheel bearing grease usually ends up hung to the race and not in the bearing anyway.