As a SMN (or SCH) you should never be running out of MP even with inefficient Aetherflow management unless you find yourself rezzing people multiple times in one fight, or you're soloing a fate for 8+ minutes.

That being said, you should also be using up more MP in one minute than Aetherflow can regen (except for certain trash mobs and then you just get your MP back between mobs anyway making Aetherflow unnecessary).