How long does it take for Mareks to kill a flock?
From what I've read and experienced, up to about 8 months old has been symptoms including paralysis. After that, it's basically wasting, but since Marek's causes immune suppression, I've started treating those skinny older chickens with general antibiotics because I am not sure if wasting should just be written off as Marek's wasting, or whether it can be opportunistic bugs moving in, such as cocci, clostridium, or e. coli.
And Marek's doesn't really go and kill a whole flock. Older birds become resistant.
You will not waste money having your chicks vaccinated. I , for one, have gotten really tired of seeing young birds get paralysis, or loss of perception and unable to aim for food. It's sad . Approximately 1/2 of my chickens have been vaccinated at hatch. The rest are older chickens.