At each rank the spell must be channeled for at least 3 seconds in order to earn back the mana invested in the initial casting.
This spell can be very effective in Arena as it can be used to run enemy healers out of mana limiting their ability to heal and support their team.
In PvE it sees situational use against specific bosses in Instances who can heal themselves. Also, it can be used against some mobs that can drain YOUR mana and use it to cast very nasty spells. By draining back your mana you stop them from performing these spells. These mobs include Wretched Devourers for example, they cast Nether Shock that deals heavy shadow damage and also silences the target. However it can be hard to prevent them from casting it since their drain is an Arcane Torrent, which is instant, and therefore you must be very quick at draining the mana out of them before they use it. Many Nether Rays also use Nether Shock or Arcane Explosions using mana drained from you by their own Drain Mana.
Because casting it costs mana, Drain Mana is inefficient as a mana restorer.
Demonology Warlocks with Mana Feed can use Drain Mana to restore their demon's mana, but not as efficiently as using Life Tap.