They don't. A preview stays open until you keep or kill it — there is no clock on it and nobody decides for you.
While a take is undecided, its Credits sit on hold in your balance: not charged, not refunded. Keep it and the hold becomes the charge; kill it and the hold returns to its original Credit allowance, with the same expiry date. Subscription Credits whose original deadline has passed expire when returned. An undecided take costs nothing and spends nothing from your kill allowance.
Until September 2026 previews expired after 72 hours and each expiry spent a kill. That rule is gone. If your allowance dropped because of expiries, it has been restored, and every take that expired that way is back in your studio as a preview, ready for your verdict.
The habit that still pays: judge takes while the brief is fresh. Not because anything expires — because your eye is sharpest right after the render.