My experience is that my genuine watches shine very brightly at first, and slowly fade ove the course of an evening.
My old school "super rep" Fiddy is absolute light sabers when fully charged, but the rate of decay is much swifter and levels off at a very dim output in just a couple of hours. It stays visible for several hours, but the illumination initially drops quickly. The dealer (no longer around) called it Superluminova. If it is, I think he thinned it too much, or used a corollary product for dollar savings, as Rocketeer mentioned.