Here's my opinion -- The bearings may last a long time if the grease isn't contaminated, but how long do the seals last? The seals are the shield between the little world inside the hub and the cold, hot, wet, dry, gritty world outside. One surface of the seal rides on the shoulder of the axle
spindle where grit may or may not build up.
If the seal fails to block foreign material (grit or water), the bearings will eventually fail. If the seal fails to block grease from getting out, the bearings will eventually fail and the brakes may become contaminated. Interestingly, I believe that the brake drums provide an extra degree of protection for those seals and bearings.
So, the question is, should the bearings-seals-brakes be disassembled, inspected, repacked and adjusted to prevent potential brake and spindle replacement or brake failure? Or should one allow time and miles to go by? We each have risk tolerances.
If one considers oneself incapable of doing these tasks oneself, one would be wise to find a local mechanic who would agree to do them on a regular, non-emergency basis for negotiated fees.
At a bare minimum, I strongly recommend that anyone exceeding Dexter and Al-Ko's recommendations at least remove the dust cover and check what grease can be seen for water contamination (turns milky).
Al-Ko brakes, bearings and seal standard service schedule, per the Owner's Manual PDF on their site, is "6,000 Miles or 6 Months whichever comes first".
Dexter recommends "12 Months or 12,000 miles" under the same circumstances, per the Maintenance Schedule PDF under the Service Literature at:
As I have said before, both of the above sites contain a wealth of information on all the running gear that may be found under most of our trailers and I highly recommend downloading some of it to files for reference either by oneself or by a mechanic.