A timer is a good idea, but I wouldn’t have it function when you don’t get hit; still a better idea than the current crit function. That works more to the advantage of classes with a lot of dodge and stamina. Like others have mentioned, some boons, curses, and sienna’s venting could disrupt the build-up too often.
I’ve mentioned this elsewhere, but I think it should function similar to bounty hunter’s blessed shots. Just have a 10 seconds cooldown between triggers and you get a guaranteed crit that causes lightning. This way it works advantageously for just about any class and any weapon.
In the current state, it’s extremely OP for crit classes and boons, but it can also be underwhelming if you’ve only got the base 5% crit chance and you’re using a slow weapon like executioner’s sword and Kruber’s longbow. A timer like blessed shots doesn’t depend much on player skill to use, besides timing your use of it, but it is a high-tier boon; it’s kinda fun sometimes getting a massive bonus.