On my 2007 Keystone Cougar, I checked the bearings for the first time at about one year. They were nearly dry! There was very little grease on the bearings, and the bearings and races were already scored with less than 1000 miles on the trailer. I replaced the bearings and grease seals, and properly packed them with grease. The second set of bearings lasted until I sold the trailer in March.
I always do an annual inspection of the bearings and brakes. Your best bet is to disassemble the hubs and visually inspect the bearings, brake linings and magnets. Don't rely on those EZ lube gadgets.