If they're set up right, not too loose or too tight, and properly lubed up they can last for years. Hard bumpy roads and/or a heavy rider will shorten their life some as well as going a lot faster than they were designed for.. I'm talking about going 35+ mph on a daily basis tho.
If you got a fast bike and ride it fast a lot then it would be very wise to check, clean, and adjust them often and I would also recommend this if you ride in a really dusty environment.
When you check the bearings the grease shouldn't be turning black because if the grease is turning black the bearings are shedding off very tiny like microscopic sized particles of metal from the balls, the races, or both. So basically if you clean and adjust your bearings,then check them after a few weeks and the grease is starting to turn black it's time to clean the races, check them very closely and replace the balls. It could be a small nick on one of the races or a ball that went out of round but best to investigate.
You don't have to check your bearings but maybe once a month if you ride in a clean dry environment and mostly on smooth pavement keeping your speed under 30mph, but if you change any of these, your environment, the type of road surface (smooth vs rough) or if you begin to cruise at higher speeds or ride faster most the time it would be a good idea to keep closer tabs on them.