afaik, the info on the wiki
HERE is still accurate. AC is always useful, but uses a hidden formula that most are unaware of, so it's often disregarded as useless. The chart there goes up to 500 AC, though
over 1k ac has been reached. However, most tanks will do just fine with 400-500 (barbarian tanks get away with much lower due to using other defenses more).
For your average non-tank though, anything over 100 is generally great. The trade-offs to get higher are often not worth the losses elsewhere (usually a loss of DPS)