If we look at Zarqawi's lifespan from the time he first became known, and divide that by a factor of 4, it's probably as good an estimate as any other.

Besides the ones that tried to stand and fight in Fallujah, we'd probably be surprised at the longevity of these guys. Their "engagements" are very relative, as they most certainly don't join the fight and stay engaged for any length of time before it is time for rest and refit.