Other breeds that run off will usually come back in no more than a few hours if they don't get hit by a car. Again, it's training versus instinct. Your Whippet must be either leashed or in a securely fenced yard every time he's out for his whole life. A 4' fence is usually enough if a Whippet is not a jumper or climber. Some do dig, but it's not too common a problem.
Security is critical.
We don't even walk a Whippet from the house to the car without a leash. It is amazing how quickly the worst can happen. And the first time a sighthound gets away from you can easily be the last. It's easy to become casual about it when your Whippet is generally obedient and calm. Remember that if he's okay off-lead 99 times out of 100, that 100th time could be the day you lose your best friend.