I'm pretty sure you spend longer hours when playing a game like I do so your PS3 should be able to handle occasional 3d viewing unless you plan to do marathons.
I would think that the duty cycle is a lot lower for gameplay, as you don't read an entire BD game from start to finish. The console just loads certain parts once in a while, and the drive remains idle the rest of the time.
BD movie playback (whether 2D or 3D) will usually need constant OD operation. That being said, I'm sure that this was factored into the design. Still, if the built-in drive's parts are like that of a typical modular OD, then it should have an MTBF of around ~60kPOH @10% duty cycle. If gameplay duty cycle is 30%, then that's ~5 years of 24h/7d use (MTBF numbers are often very optimistic, so I'd probably expect 2 years average... or 6 years for 8h/7d use). Movie playback duty cycle is a lot higher however, and may cost twice to thrice in terms of drive life.