Actually, for bearing manufacturers, bicycle applications are about as easy as they get. Low speeds, no temperature extremes, and light-to-moderate loading (compared to other applications), used only for short periods of time on an occassional basis, and typically not subjected to severe environmental conditions. Even low-quality bearings will last for a fairly good while, though eventually they will fail as fidbloke pointed out in response to jimmullers observations. If the surface hardening isn't deep enough, for example, the loading will impinge through the surface hardness. I've seen the surface flake off of bearings.

Good quality Grade 25 bearings should last a long time if preloaded properly and lubed properly. There's no need to stick to only one production run either, mixing different production runs isn't a problem--we're not running them in gyroscopes....

A common bearing made today by a reputable bearing manufacturer will perform far better than most bearings made up through World War 2, manufacturing processes have improved that significantly. Of course, there's still low-quality or counterfeit bearings out there which can fail miserably.