My bad...according to my reference material, (which you've already discounted). The crusades resulted in roughly 1.5million deaths. That's a little off your number of 12k, but hey, lets stick with biblical accuracy for sake of arguement. There's no mention of length, but I was always lead to believe that it stretched over many years. I'll see what I can find.

[Link: users.erols.com]

edit:

Ok..., the conflict in the middle east is being accepted as the longest "conflict" in world history, 2000+ yrs. I think that one takes it...lol. Of course this hasn't been constant battle, but either side hasn't been at "peace" with one another for that length of time.

The first crusade start circa: 1095AD (AD added for your reference)
The final crusade (#9 against opposing religious groups) ended in: 1272AD

Again...not constant battle, but just a little shy of 200yrs for the church's violent intolerance for a difference in religious belief, just the same.