Sea turtles can hold their breath for several hours, depending on their level of activity.
If they are sleeping, they can remain underwater for 4-7 hours. In cold water during winter, when they are effectively hibernating, they can hold their breath for up to 10 hours. This involves very little movement.
Although turtles can hold their breath for 45 minutes to one hour during routine activity, they normally dive for 4-5 minutes and surfaces to breathe for a few seconds in between dives.
However, a stressed turtle, entangled in a ghost net for instance, quickly uses up oxygen stored within its body and may drown within minutes if it cannot reach the surface.