Mana pools empty at the end of any phase. There may be steps between the phases where mana stays in the mana pool.
Basically, there are 5 occasions where the mana pool empty and mana burn occurs:
- After the draw step ends (before the first main phase)
- After the first main phase ends (before combat)
- After the "End of Combat" step (before the second main phase)
- After the second main phase ends (before the End of Turn phase)
- At the end of the turn.
So, if you float mana during your upkeep, you have until the draw step ends to spend it. Similarly, if you float mana just before you declare attackers, you can float the mana until the combat phase ends.
I know it's uncouth to dig up old threads, but this is a top search result on Google and is now incorrect with the Magic 2010 rule changes. I thought it might be confusing to new players, so I'd cite the new rule:
500.4. When a step or phase ends, any unused mana left in a player’s mana pool empties. This turn-based action doesn’t use the stack.
The important difference between this and the original posters being that it empties at the end of STEPs as well now.
Infraction for deliberately and knowingly breaking the rules. Thread locked. -Woap