back in the seventies when i was racing 125s and 250s both mx and road i would just change the bearings regularly before they got a bit rough never bothered repacking them as the more grease you put in the more drag that they have but used to give them a squirt of oil. when i got back from a meeting after the bikes were washed they would be pulled completely down and cleaned from stem to stern generally on the monday morning before the workshop got too busy and check every thing like all bearings and bushes not a huge job then get the first year to give it all a good wash and re assemble generally monday night frame wise that is and the engines would get checked later in the week and rings or pistons replaced as needed. in the scope of things wheel-bearings are very inexpensive items.
i would say check and oil weekly only need a little of some good oiled if the seals aren't damaged you won't have any problems. Oh and by the way std sealed ball races will function on a conveyor belt with just a tiny amount of grease added like a pump every couple of days for ages.
