I went to a half-PuG the other night that was keeping two drakes (Tenebron + Vesperon) alive. We killed Tenebron without much trouble, then half the raid got wiped out from a combination of Twilight Torment and a Fire Wall while Vesperon was around 40%. The remaining 14 pressed on, thinking it would be fine if they could down Vesperon. They managed that and slowly worked on Sartharion, but after (what felt like ages) a while one of the healers says "Did Sartharion just enrage?" over vent.
I didn't think too much of it since as far as I knew he didn't have an enrage timer (DBM doesn't have one, dunno about Bigwigs) but shortly after the entire raid started getting hit by Pyrobuffet. Ultimately we wiped on Sartharion's "10% enrage" when the huge wave of adds come.
I'm not sure what his exact hard enrage timer is, but he certainly does have one. Our attempt lasted around 16.5 minutes and the highest Pyrobuffet debuff stacked to 8, so he was enraged for a while before the damage became unhealable.
/first to die to Sartharion's enrage

