I have not had my kegging system for long but have had a good amount of use from it already. How often should all the o-Rings or small little black seals be replaced? They seem fine now but I would hate to have issues later just because I didn't keep up on the maintenance.
Keep a few spares on-hand. When a keg starts to leak track it down & replace the problem o-ring. A leaky o-ring won't cause too much loss. You can slather some food-grade silicone on a seal to keep it from leaking if you don't have a replacement on hand. Keg-lube (or the generic food-grade silicone) is invaluable.
It's also a good idea to remove, clean and sanitize them before filling a fresh keg.
I try to remove all of the fittings clean them and put them back together every time I fill a keg. In reality, I end up doing it every second or third time. So far this hasn't bitten me on the butt, but I know for a fact one day it will.
As for replacing o-rings; I replace them all whenever I get a new keg. From there, I just keep an eye on them and replace any that leak as Dean said above.
I saw this question as a related one to one I just answered.
I'd suggest replacing the post/dip-tube o-rings every time you disassemble them. Folks have a tendency to overtighten the posts, pinching them, thus making the o-rings unreusable. The lid o-rings, unless pinched by not paying attention, rarely need replaced. The o-rings, purchased in bulk, are quite inexpensive from an industrial supplier like McMaster:
DipTube: ID 5/16", OD 1/2" (AS568A Dash Number 109) McMaster 9452K172 Buna-N $2.21 per 100.
Post: ID 7/16", OD 5/8" (AS568A Dash Number 111) McMaster 9452K23 Buna-N $2.48 per 100.
Lid: ID 3 1/2", OD 4" (AS568A Dash Number 417) McMaster 9452K218 Buna-N $12.69 per 10.
If you have a restaurant supply store in your area, I'd suggest getting a food grade silicone spray or squeeze tube - it doesn't take much of either to make a good seal and is generally cheaper than at the online stores when you figure shipping in. You should re-lube every time you loosen a fitting with an o-ring.