Well, Disc is currently mana negative, which means that doing your rotation will eventually deplete your mana. That said, there are ways to prevent this from being a real problem.
First off, even though Oracle is the recommended hero talent build, Voidweaver is perfectly viable and is much more mana efficient due to having more mana regenerating pets. It has better healing profile (more bursty) and also does a ton of damage. I finish most of keys with ~35k DPS at 265ilvl. More DPS = Mobs die faster = less healing required.
Second, some builds don’t even use Shield Discipline (talent that gives you mana when PW:Shield is consumed). This is especially important because with the Apex talents you are getting shields on the whole team all the time.
Third, use SW:Death to snipe low HP mobs (sub 20%), this will give you an insane amount of mana back. And make sure those mobs have SW:Pain active.
Other than that, before doing any keystone, make sure to enter a follower dungeon to get some free Mage food and try to drink in between pulls. Usually what I do is run ahead of the tank when the trashpack is dying, so when the last mob dies I can drink and get 2-3 ticks before I have to start healing again.