When does Armor reach a pinnacle? (in General)

I know this is probably the most vague question of all time, but right now my Wall has over 300 armor. Is there a point in armor rating that would make improving it ultimately useless, or is there no such thing as too much armor?

this link http://www.carnageblender.com/wiki/Armor+Class will help explain it better than i can. but iirc there is no hard cap just when the + get to be too expensive

because of the nature of armor deductions if you were able to reach a 500 you would have 100% damage reduction. That would be silly. So there are built in limits (kind of) to prevent that. In my mind it is a bad model. I think it should follow a reduced gain model. I would have structured it as for every 10 AC you get a 1% reduction multiplied in. In other words a 50 AC would end up being 99% * 99% * 99% * 99% *99%. As complicated as that seems there is a nice formula for that that would not require that you multiply by 99% that many times. But following that model you would NEVER reach 100% damage reduction and each increase helps but with diminishing returns.