It lasts a number of rounds equal to 3 + Con, just like normal rage. It's just that normal rage increases your Con, whereas this one doesn't.
I don't think this is a huge downside at all. I think I've had 1 or 2 fights in the last 18 months of play where the barb's rage wore out before the end of the battle; generally the fight is over _long_ before then. Frenzy is otherwise probably just as good, if not better, than normal rage. The extra hp from normal rage is a mixed blessing; if you actually take enough damage that it matters, then you'd better hope there's a cleric nearby, because if not, you'll go down when you stop raging. Normal rage also gives you a bonus to Will saves, but you also get a penalty to AC, whereas frenzy gives you a bonus to AC.