I found a link to the EZLoader site for trailer manuals that can help to answer your questions:
http://www.ezloader.com/Manual.asp
(Go to page 26 for the Oil Bath Lubricaton System)
I can't find a specified referral to changing the oil for mileage, but it stands to reason that "it depends...". If your hub oil turns murky-brown , it indicates the water has found a way in, ...and that would be a very good time to change the oil (and probably the O-ring). EZLoader does say in the manual that you should.. "check the bearing ADJUSTMENT every 2,000 miles or before every season, whichever is less." (page 27).
They also say .." Note: Bearing Lubricaton Systems are not a replacement for proper bearing maintenance." (page 26).