That would probably take along time as someone who experienced a lower leg atrophy it took 1 1/2 months of physical theropy to solve yet there is still lingering damage from it weakness and pain that will last the rest of my life. So given my own thing its possible a long coma like that could take a year or slightly more they're PTing their entire body which takes time and a lot of effort some parts might work better then others like they may always be 90 - 97% weaker in the arms then the legs or something.
This all assumes these rescuers want to spend so much time on an invalid remember they didn't get basic pt in bed so they shouldn't be able to move anything at all no matter how hard they try. The rescuers need equipment, trainers, and know how to fix all of that considering you want him to be as good as he could be as he will most likely not be 100% anymore.
This person I'd imagine was also neglected if malnourishment isn't also an issue deficiencies in vitamins might be which can cause a host of issues including organs not working properly. There also might be anxiety mental or physical or both. A physical anxiety is body pains, weakness, breathing issues all with little to no cause striking really a ton of things.
This person also has trust issues are these people really the good guys? They have kidnapped him from the other but that doesn't mean they're doing it for noble reasons.
There is also the uncertainty of where is his family I'm assuming he as people also deemed undesirables? What if he decides to not work with the people who saved him until they're released how important is he to their effort? Does his rescue and recovery time justify who he is and what he's expected to do in return? Like I got why Shepard was rescued even though the science was flimsy but I thought it was too soon a revival and being the symbol for humanity kind of sounded like a lame excuse for an undoubtedly godly expensive project.