Our newest trailer is 3 years old, towed over 15K miles. Just pulled the wheels to regrease and IMHO there was no real reason to regrease, although I did. Plenty of grease, no moisture, bearings in good shape, so other than checking the brakes etc. there was really no reason to do anything. My other trailer is 10 years old, bearings never repacked, has about 25K miles on it. will pull them this fall to check and compare. I'm inclined to let them go 4 years at least , 25K miles before checking again. I still don't understand why trailer bearings need repacking so much more often than similar bearings on cars, which will go 100K easily.
My neighbor is pulling a 28ft cargo trailer, has about 150K miles on it in the last 10 years and he pulled the wheels and replaced brake shoes at 100K, said bearings were still in good shape and plenty of grease.