"Properly maintained hubs should last almost indefinitely. Even ""classic"" 1994 hubs have a very good seal, so unless you ride in constant wet weather or on the beach :-) one could limit a repack to once a year. The issue with hubs is more of contaminated, dirt grinding grease than a lack of it.

With each repack, it doesn't hurt to replace the bearings. It'll cost you less than a 5 spot to do so for both the front and back. Another item that will need to be replaced eventually is the free hub body.

The real limiter to the wheel life expectancy is the rims.
Each time you apply the brakes wear occurs against the
rim's sidewall surface. How quickly one wears through rims involves alot of variables like rider's weight, how often you brake, brake pad quality, riding conditions (e.g wet/sandy), the weight/sidewall width of the rim to name a few.

To answer your question, you or the bike shop need to remove the tire and tube and inspect the rim to determine how much wear has occurred. New equipment (wheels) are much cheaper than doctor's bills, so there's no sense in squeezing out every last mile of the wheels. Naturally, use your best judgement."

my own experience has taught me that as one item mentioned in earlier post, the rim side wall it the most critical area that can cause the most damage. Learn what the mfg. spec for the sidewall thickness is, then with a dial caliper measure the thickness that the sidewall is currently. Once you hit the 50% mark of wear, it may be time to replace what may look like a perfectly good rim.
While on a ride in Pa. on a downhill, a corider's rim blew when the sidewall was just worn too thin to handle the brakes on the decent. Luckily, he escaped any serious injury, which I attribute to luck.

Good luck