i dont think anyone knows forsure because every concussion is different. my son at 16 had his first concussion may 2014. he was never officially diagnosed with concussion and kept training through it. he never played contact after the hit but kept skating and working out. never shut down with complete rest. by august 26 2014 he just couldnt do it anymore. headaches, dizzy, etc all got way out of hand. aug 29 2014 he shut everything down. he has had many treatments and it only seems that this complete rest is allowing him to heal. 5 months after complete shutdown he is seeing a light at the end of his brutal pcs. not there yet but having some good days amongst the bad. it is hard to know 100 percent if it is the rest or just time and had he continued to train if he would still be in the same place now. he does know that on a daily basis the rest allows him to feel ok and not suffer horrible headaches daily. that has been a relief. he has gone from advil daily to advil once or twice a month.