Per the
Dexter Axle service manual your bearings should be lubricated every 12,000 miles or 12 months.
Two schools of thought on the axle zerks.
One - DON'T DO IT!
Second - it is ok to use the zerks as long as you do it carefully and follow the Dexter instructions exactly.
It is a good idea to pull the hubs and inspect the bearings and brakes regularly. And the bearings can be hand repacked at the same time.
I did use the zerks on mine and had no problems at all. But do not consider that my recommendation to all.