I often refer to the concept of you being your dog’s trainer. With that title comes great responsibility, a little basic knowledge, and a whole lot of common sense.
If you were my trainer and my goal was to get myself stronger and build muscle in my biceps, would you have me start with 50 lbs weights doing curls? No, you would have me start with 20 lbs weights and teach me the importance of form and what it means to lift weights properly and safely. Once I had good form and lifting 20 lbs was easy for me, you most likely would then have me lift 25 lbs and then eventually 30 lbs, etc. You get the point.
The purpose of this analogy is because we all have had our personal experience with physical exercise or physical therapy for that matter. The same principles apply to your dog’s recovery. You need to establish your dog’s baseline and then slowly, carefully, and consistently challenge them to get stronger.
So the simple answer to the above question is when your dog is doing the current exercise perfectly, consistently without any signs of excess stress, then they are ready for you to increase the difficulty of that exercise.
That being said I strongly encourage pet owners not to take the leash off of their dog until they are 100% positive that the dog is READY to run and play.