It depends on how many miles you ride, what kind of roads you ride on, what weather conditions,
The wheels will spin freely on the bike well past the time the hubs should be repacked. Take a wheel
off and turn the axle gently with your fingers. It should be smooth, with very little drag. Expect
some drag- that shows the presence of good grease. An axle that spins with no resistance indicates
too little or dried up grease.
If you're mechanically inclined, rebuilding hubs is quite easy and requires minimal tools. I usually
rebuild mine every year or two.