How does mana regen work in Cataclysm?

Solution 1:

The "five second rule" no longer exists in Cataclysm. You have a fixed rate of mana regeneration while in combat, and another fixed rate of mana regeneration while out of combat.

Out of combat

Your out-of-combat mana regeneration rate is based on your spirit. "Mana per 5 seconds" (MP5) no longer exists on items, and has been replaced by spirit. (You'll still see it on mana oil, though.) Every class that has a mana bar will regenerate mana this way while out of combat.

In combat

All classes start with a baseline regen rate of 5% of your base mana per second. (Base mana is the amount of mana you would have with 0 intellect.) Classes using healing talent specs gain a passive ability called Meditation, which lets half of your out-of-combat regeneration continue while in combat.

For non-healing specs such as mages, warlocks, and shadow priests, spirit does absolutely nothing for your combat regen. Many talents and abilities exist that give you bonus mana in combat, however: Replenishment, Archangel, Hymn of Hope, Shadowfiend, etc. Mages can passively regenerate mana using Mage Armor.

Solution 2:

WoW wiki has a great article:

http://www.wowwiki.com/Mana_regeneration

Little summary:

• No more 5 seconds rule. Replaced by in and out of combat.
• In combat regeneration: fix amount (5% of base mana) + talents (like mediation up to 50% of "out-of-combat regen")
• Out of combat regeneration: scale with spirit and intelligence.
  • To double regeneration, either double your spirit or quadruple your intelligence.