When you pause or delete a campaign, the system will automatically refund its remaining balance after approximately 20 minutes of inactivity. The only exception to this rule is push traffic, where it takes 24 hours.
More info about push ad campaign refunds: Due to the potentially long delay between a push ad being served to an end user and them clicking or tapping on it (when they check their phone or desktop computer's message center) push notification ad campaigns have a 24 hour delay after they're paused or deleted, before they will auto-refund their remaining balance.
Learn more about buying push traffic here.