When it comes to any kind of injury, it will depend on the type of injury and the care that you needed after the injury occurred I find. 

For something that was small and that I could deal with at home, I usually give myself a few days and wait until I feel I can then workout or train again without any pain or discomfort. 

For something such as a bad injury that needed me to get hospital treatment and such, I will go by what the doctor says as to how long I should wait before I jump back into anything.