Question about Mareks disease in Chickens - mortality time?

I bought 2 hens at auction, the box did fall on the floor during the sale.. got home, they weren't moving as normal - a few days later one was walking, the other wasn't. The non-walking one just sort of sat. Her legs had power, but she wouldn't "stand" on them.. she did sort of shuffle along.

Noted the rooster was also picking on her so brought her into the house. Started her on vitamins (selenium and Vitamin E) and electrolytes.

No improvement.

Finally got contact with the seller who says both birds (8 month old silkie hens) always acted odd. Great.. so they sold cull birds at auction... he says he thought they walked but that they never came out of the coop.

No eggs from either..

Anyhow, so over the past 2 weeks I note she isn't getting better and is getting worse: specifically flopping to one side which I did see as a symptom of Mareks. She can move her wings and legs, she is not paralyzed.

The area vet knows nothing about chickens - only cattle, horses, etc.

Anyhow..

TLDR - if a chicken has Mareks how long would it take from them showing symptoms to dying?

I'm wondering if she was hatched faulty, got injured in the fall, or what?

EDIT - adding I have had them both for a month, one is normal (except no eggs yet).