Read more! You should be able to find prognoses for common injuries like muscle strains etc no problem, then its just a case of knowing your physiology, inflammatory process, stages of healing etc. Lots of it just comes with experience though mate once you've seen 5 or 6 hamstring strains you'll know when one is better or worse, and roughly how long you take to rehab them. add a bit of time for previous injuries to the area, use your clinical judgement to know when a player needs a more safety first approach to their rehab. Also talk to the player. if they've had something similar before they probably know how long its going to take them to recover and its just a case of guiding them through the process