Asking when you can allow your dog to be off leash is not a whole lot different from the question, "When will you be able to let your son or daughter take your Porsche or Mercedes out for a spin with his buddies or her friends?" The answer can range from now to never. Different circumstances would dictate different answers as well.
Most adults would go by this rule of thumb: When the individual is responsible and trustworthy enough for you to have the confidence that he or she will not bring harm to him or herself; others and of course the car. You can apply this same principle to your dog.
Is your dog socialized enough that he or she will not be fearful of or aggressive towards other people and dogs. Can you trust your dog not to jump on people (especially children), chase joggers, fight with other dogs, pick up garbage, invade picnic lunches and so forth? Unsocialized and skittish dogs will often bolt if something frightens them.
Can you control your dog off the leash? Will your dog reliably come when called and stop on a dime from a full run when told to stay? These commands are essential for your dog's safety. Some dogs when let off leash will simply run away. Other dogs will chase a tennis ball or cat right into the street.
If you are willing to risk the safety of the public, the safety of your dog and the security of your finances (paying your own or someone else's medical or veterinary bill, facing a lawsuit, etc) then you'll let your dog off-leash before someone who is not willing to take the risks.
If you are unwilling to take the risk, only let your dog off-leash in areas where the above mentioned risks do not exist. A fenced-in dog park is ideal. Tennis courts are usually completely enclosed. Your dog may not come when called but at least you can just go and get him or her when it's time to leave. If your own yard is not large enough, find a friend or neighbor with a yard where their dog and yours can get together to run and play.
Use a long leash on outings to give your dog some freedom but still allow you to maintain control.
Train, practice and be patient. Obedient, trustworthy dogs are a product of a lot of dedication and commitment.