The EZ lubes on my 21 ft TT tandem axle are spring loaded cups that basically are a grease reservoir under pressure, pushing grease at all times towards the back side of the axles, and the seals there. You can watch the cup move outwards radially towards the grease gun as you pump them up, taking up the spare spring space. I fill mine with the grease gun every couple of years. I managed about 20,000 miles from when bought new to just this last April when I hit them for the first time with the grease gun.
How far do you drive on your car bearings without repacking them with grease? Same principle.