I'm interested in this info also. I bought a bunch of used kegs recently and was told the O rings needed be changed on some, but they haven't seemed to have any problems yet.
Only when the keg is new (to me that is). If one of the post o-rings gets worn I would replace it but the lid and dip tube rings I've so far only changed when I first got the keg.
It's not a set time - just look at the o-rings every time you empty or fill a keg and replace one if it is worn out. If it has cracks or other signs of wear, toss it. If you have any doubt about it, toss it.
I replace all mine as soon as I buy a keg. The old orings were used with soda, plus don't want any chance of leaks. Its a small price to pay to make sure you don't have a beer failure.
I replace all the gaskets when I buy a new keg (gotta get rid of the soda contamination). In the 18 months I've been kegging I've only replaced a couple of dip tube gaskets due to wear.