I have Dexter EZ-Lube axles. Per Dexter they need to be lubed once a year or 12k miles. I am at my year mark. I have a boat that requires lubing several times, but has a cap that pushes out letting...
Make sure you have the right grease too. It needs to be compatible with the old and also thin enough to flow thru the small orifices. I have them on my axles but have never lubed my bearings that way. I just did a tear down of one axle at a time. Checked out my brakes and anything else you can't see when the tires mounted. I will go 2 years before doing it again.