Which weapon?

[Nix80]

is that true? ive always read it as .. increases your crit rate, for every spell that dosent crit, increases crit rate. i dont think ive ever seen it last for more than 2 spells.

No It has nothing to do with how many spells you have casted.
While doing AV I always pop Excruciate when entering one of the poison rooms, by the time we get to the spot and I cast Flare the timer is at < 10 sec and is always a crit