When you use Tree of Perdition:
Defender
{T}: Exchange target opponent's life total with Tree of Perdition's toughness.

Your opponent has 20 life before you tap Tree of Perdition. Assuming you tap this creature round 2 against a player and now the player's life total is 13, during your next turn, during your untap phase, you untap Tree of Perdition, does the player go back to 20?

I was told if it doesn't specific saying "until end of turn" then the effect stands. Where in magic should we assume this rule. Why wouldn't the assumption be "as long as creature is tapped" does ability with stand.

Thank you,
Jay