You need to start reading about Mareks. It is a complicated disease, and there is no definitive answer to your question. This is a virus that can do MANY different things, and can hang out in the environment for years. The classic/most common presentation is the one leg lame bird that progresses to both legs and death, the bird becomes ill usually around 4-5 months. It can happen sooner or later. The other presentations- visceral, feather follicle, ocular- can take longer to make the bird ill. Time of death from first obvious symptoms can be days to weeks to months (or years sometimes)- it really depends on how much supportive care people put into these individuals. Given no extra care, a bird that can't walk will die from exposure or dehydration within a week. If put in a cage with food and water placed right in front, kept warm, their bottoms kept clean- these birds can linger for weeks or months. It would be helpful to provide why you think your bird may have Mareks, age, vaccination history, ect.
