Since switching to Arcane (which I love) from FFB, I've been doing a AB x3 + Amissile(when it's up). I occasionally use Arcane Barrage if the missiles don't come up for a while to reset the AB stack and conserve a little mana. On the longer boss fights I always come close to running out of Mana, but never seem to before a cooldown is up. I do use mana pots much more than I ever did with FFB, though. If it's really close, I'll do ABx2, then whatever is available-either Arcane Barrage or Missiles. Then when I top off, I go back to the ABx3. But, I'm still experimenting with the rotation just to see what the best DPS is. Ever since I switched to arcane, my DPS has been so high, I can afford a little experimenting here and there.
The mana depends a lot on who is in the raid. No replenishment hurts some.