Originally Posted by
Kaysha
This is actually only valid in raids. During Heroics, most boss fights seem to last like 3-5 minutes, tops. This leaves only room to pop it once effectively. Popping it almost around the time of the pull means you lose out on its mana regain, since you're still topped. Popping it right as the boss dies is also pretty much a loss of mana regain, as you won't be using much of it, if at all.
During raids you will have to figure out how long a particular fight takes and pop it whenever you see fit. Most boss encounters seem to take between 5 to 10 minutes; either early on so you can use it later again, or somewhere midfight, when you are burning mana. This is mostly a gut-feeling, though, at least to my personal experience.