I haven't had an unvaccinated cockerel last longer then 24 months. They either die outright from breeding stress at point of crowing or around their second birthdays from secondary Marek's tumors or infections. I have lost several to seizures and neurological problems at 8 or 9 months. I never had a necropsy done on my birds but have talked to an avian vet at Missou's veterinary lab who confirmed that it sure did sound like I had Marek's in my flock without even a necropsy. Not long after I lost my first 9 week old chick to classic Marek's sissor paralysis and noticed a rooster and hen with grey eye. My husband is a retired eye doctor so I asked him to use his portable slit lamp and examine their eyes for me. He did and confirmed that three had severe herpes infections in their eyes.
No need for a necropsy at that point in time. I had my diagnosis.
I have two vaccinated pullets in my flock now. Bought at Orschelns to keep a single chick hatch company. The chick died....yep, Marek's but her two little coop mates are going strong.
I'm hoping to add a few more vaccinated bantam chicks to my flock this next spring. In the mean time, I waiting to see who is going to survive so I can start to breed for resistance.
The magic number for my flock is 24months. So far I have hens that are 34 months old. I just need a good resistant rooster now and I'm in business.
Good think I like roosters. I sure have a lot of them that I'm hoping to get one or two good boys to help carry on my flock for me.