It takes 1 hour to regenerate 10% of your mana if you are any class other than magician (with a minimum of 5). Magicians regenerate 20% mana per hour(with a minimum of 10).

To increase your mana you have to use your points you earn by leveling up. each point adds 5 to your maximum mana.

There is no way to add mana to your pool other than by waiting for it to regenerate (eg. no potions)

It is possible to drain a players mana with the spell mana drain, which drains half of your opponents mana, but does NOT give you their lost mana.