Hub oil should last for 3 to 5 years or 30,000 miles to 50,000 miles.
Many over the road semi trucks run at least 100,000 miles before changing the hub oil.
The hub oil should remain in the hubs for all that time unless a wheel hub seal goes bad and leaks the hub oil out.
It's best to check the hub oil level and seals at least every few months to 6 months to ensure that the hub oil isn't leaking out and if the hub seal is bad or going bad it should be replaced.
If the hub oil all leaks out it will eventually cause the wheel bearings to seize up.
If you do a lot of hard braking or towing you may require more frequent changes of the hub oil.
Off road driving or driving in extreme temperatures can also affect the lifespan of the hub oil.
The kind of oil that you put in hubs is 75w 90 gear oil although some hubs may use a 50 W gear oil or other hub oil.
When adding the 50 W or 75w 90 or other gear oil to the hubs you add the oil slowly until the oil touches the full mark on the wheel hub cap.
It can take awhile depending on the size of the wheel hub cap and wheel hub.
After you fill the wheel hub cap with oil to the fill mark, give it a couple of minutes and recheck the oil level and if needed at some more slowly.