When I was investigating boots, I found their durability loss formula to be 1% * [(base damage) - (boots armor rating)] (with a minimum of 1% per hit). I would guess armor works the same way, but I haven't tested it.
What does this mean for necroarmor? If an archvile hits you for 12 damage (average according to the wiki), the armor blocks 6 of it, and takes 1% * (12 - 6) = 6% damage. The armor would subsequently take 6 health to recharge (the wiki says its 2 health per 1% but I suspect the editor confused health with %health). For larger hits, the necroarmor can even give you a net loss of health compared to NO armor...although the damage is spread over time. For smaller hits, the necroarmor fares better, but still isn't that impressive especially because of the aforementioned 1 damage minimum.
Of course, this is making the assumption that I understand how the mechanics work.
EDIT: I don't mean to say the necroarmor is never useful... rekenne's suggestions are good.
EDIT 2: Oh, and the necroarmor will stop regenerating itself when you are at 1 health.
