If you are using a strong PVA wood glue, you should leave your project in clamps for 30 minutes to one hour, provided the joints are not stressed. If the joints are going to be under stress, it is best to leave the project in the clamps for 24 hours. Polyurethane glue requires less time in the clamps - the recommended time is roughly 45 minutes. Be sure to take the climate, wood type, as well as the manufacturer's instructions into consideration.