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...
The hub is designed for the old grease to be pushed out the front of the hub as the new grease goes in the rear of the hub. That is how it purges the old grease. There will be a lot of old grease come out the front before you start seeing new grease. Look at a cut away view of the hub. Or watch the utube video.