When do bearings need to be replaced? How do I know when bearings need to be replaced?

Should I go by time? Or is there a symptom to guide me?
I refurbish a lot of older bikes for re-sale.  The first thing I do is to pop off the wheels and manually turn the axles to see if the bearings are suspect.  If the axle turns roughly, has side-to-side play, or is hard to turn...They probably do.   

There should be "no more friction than supplied by the grease" when turned by hand.  You cannot do this with the wheel on the bike.
If you have the tools; cone wrenches and such, then a wheel rebuild is both easy and cheap.  The best-grade #25 bearings are inexpensive.   If the cones are shot it's a different story.

Time?  My police patrol bike, with well over 3000 miles, still feels perfect.  If you ride in the rain a lot, or ride off-road through mud and sand and through water crossings, naturally you're going to need more frequent maintenance.
Bearings are something to check say...Yearly for most riders.