Direcharm lasts, in theory, forever. But the max you'll be able to stay in most zones is 45 minutes if you push it, due to the packet loss bug (which most people should be familiar with).
That and a distinct lack of appropriate level 50 mobs for charming in most zones, led us to look into an alternative AA's (AAA?) to replace it with.
Side note for those who never liked Enhanced Control, it's actually not out of the realm of possibility to have Dire Charm return someday as regular spell, rather than an AA. It would probably get just as little use that way though.