Yes, Marek's can advance that quickly. It can even cause sudden death or a bird can linger for months or even suddenly and miraculously recover from a seriously debilitating outbreak like this, in a matter of a couple of days. There are no hard and fast rules with the disease, just a huge variation in terms or time and severity as well as type of symptoms. As long as they are still eating and drinking there is hope. Keeping them as happy and stress free as possible is important.