Normally, each player's mana pool empties at the end of each phase, step, and turn. Untapping permanents happens during the untap step, and no player receives priority during the untap step, so this means that your opponent would have to tap their lands on your turn, in which case the mana would empty from their mana pool before they untapped.
There are a few cards (like Upwelling and Kruphix, God of Horizons) that prevent your mana pool from emptying. If they had one of these cards in play, they could tap their lands for mana during your turn and then save it until their turn (or for as long as they liked).
If I had Kruphix or upwelling out, could I tap my lands during my upkeep then retap AFTER the untap step?
I was curious as the other day, My friend tapped his lands for mana, untapped them on his untap step, then re-tapped them for more mana. I was wondering if that was even legal in the standard rulings. Please help?
If I had Kruphix or upwelling out, could I tap my lands during my upkeep then retap AFTER the untap step?