Here's what I did:
1. Cast Summon Elemental and Sonic Boom. This summons an elemental and turns it hostile.
2. Fled to a different area.
3. Rested 8 hours.
4. Returned to that area.

The elemental was still there (and it didn't go away after the first round of combat). Shouldn't the spell have worn off by now?

Edit: Using Turncoat allows the battle to end a few turns later, at which point the spell finally wears off, due to it being a combat only spell.