The ability holder will wake up in half the time, rounded down. If the turns it would sleep for 6 turns, it would wake up after 3. If the Pokémon is only supposed to sleep for one turn, they will wake up at the end of the turn. It also applies to self-induced sleep from Rest, reducing its length from 2 turns to 1 turn.
Pokémon with Ability
As a Hidden Ability