I currently have Marek's in my flock. There's been birds with leg & wing paralysis, ocular, external tumors, wry neck, the works.
Each strain of Marek's is different...manifest differently... differences in severity...
In my flock, it can be 0 to 7 days from onset of symptoms to death. Most recently I had a girl that was just starting to show leg paralysis, I brought her in side and she seizured and passed while I was on the phone with the vet.
The one that presented with ocular also had tumors. He went through 7 days of antibiotics and vitamins before I gave in and had him euthanized.
I had a 17 week cockerel that developed wry neck. We tried vitamins, antibiotics, the works... Once he list interest in eating and drinking, I euthanized him.
I had birds that presented with pecking coordination problems. They will either have sudden death in 3-4 days or will develop paralysis and go for a little longer before I euthanize.
This is all from this year. I April I got six 3 week old cream Legbars and icebars from a local breeder. When they hit 17 weeks, symptoms started. I list all 6. During that time I had two broody hatches on 4 another with 7. I have lost 4 of those. From my previous year flock of 23, I have lost 2.
Once symptoms presented, I have had no survivors.
Denial teaches you a lot in observation, but it comes with the cost of suffering. Don't let your birds suffer.