When a druid spends his last hold, how long before he turns back into a human? I'm guessing it needs to match the fiction. So if he was a bear and he did the "Bite and Claw" he would be human shortly after the attack, but if he was a duck and did the "Fly Away" he'd stay a duck until he finished the flight (however long that might be).
The GM will also tell you one or more moves associated with your new form. Spend 1 hold to make that move. Once you’re out of hold, you return to your natural form.
The short answer is when you run out of hold, fictionally after you've spent the hold to complete the shape-specific move. While you're shapeshifted, though, you can act as that animal without necessarily using moves. In your duck example, I could see having a Fly Away move, but just flying somewhere isn't necessarily a use of the move (which, by its name, sounds more about escaping an attack). You're a duck, so you can fly—no move required.