It's easy to put grease in new equipment, but harder to know when it's time to regrease. You should follow the manufacturer's recommendation, but if that's not available, the following formula can be a reasonable estimate.
If you have a trailer and take off-road vechiles to the desert, sand contamination is your enemy. If you have a trailer and put a boat into the water (fresh or seawater), water is your enemy. When trailer bearings fail, the balls can become hot little ignition sources spitting out of the axle hub. When the risk is high, take a more conservative Contamination Factor, inspect frequently, and repack your grease at the first stign of trouble.
Equation source: Practical Handbook of Machinery Lubrication, Second Edition