How long is a piece of string?? Some only survive a few days, others can go months or even years. It can depend of the strain of Marek's, the bird's immune system and their circumstances. If they are happy and well fed with no obvious stressors, they can survive longer and may even regain some muscle control and improve. I've had some go lame one day be floundering on the ground the next day, unable to stand up and then 2-3 days later you would not know there had been anything wrong with them and be fine for a whole year before having a second attack. Others were nest bound and took weeks/months of TLC to get back to being able to range with the flock but still retained a limp. Some deteriorate over a period of days, weeks or months and either die or are euthanized. I just despatched a cock bird this afternoon that came down with it 10 days ago. He couldn't stand or walk and started to refuse food. Once they won't eat, I end it for them. He was skin and bone and if I get a chance tomorrow I will open him up and see if I can find any tumours.