The thing to remember is that NOTHING has really changed in small trailer wheel bearings since the '70s except the introduction of a better seal (has two lips and a spring retainer instead of one lip and no retainer).
Recently the axle
manfs have come out with systems where one can slowly handpump fresh grease through both bearings, pushing the old grease out ahead of it (not perfect, some folks have had the seal blow and pumped grease into the brakes
...; and not to be confused with Bearing Buddies, which are a different story as search of group should reveal).
HOWEVER, the manfs still recommend disassembly, clean, inspect and repack annually.
Some folks have gone much longer than that and some found out the hard way that they went too long (Voice of Experience here -- That "I don't even know it's back there" trailer kept on being pulled until the bearings on one side welded themselves to the spindle, ruining the shoulder where the seal rides and requiring complete axle
replacement -- Immediate inspection of other side revealed the grease was water-contaminated).
Things have changed in larger wheels (six lug, 16") with oil-filled cartridge bearings -- If they start to get warm, replace them. Maybe someday they'll bring that technology to the small lane.