How many times does your mana pool empty per turn?

According to Rule 163 and multiple RiftJudge FAQs, your rune pool (which holds your Energy and Power) empties twice per turn:

1. After the draw phase: This is to prevent exploits where players could float resources from the beginning of the turn to use later.
2. At the end of the turn: Any remaining unspent Energy or Power is lost.

As noted in Rule 163.1 and FAQ #1497, these are the only two times the pool empties; it does not empty during cleanup steps or after combat.