This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Because there are so many different ways to respond to the presence of the virus, it is impossible to predict exactly what will happen (and when it will happen) to your cat. I've noticed that any 5 vets you ask will give you 5 different answers to this question. The scientists who experiment and publish on the disease are also in disagreement. I have presented two different (published) viewpoints below. According to Hardy, et al (1980), (and these people seem to be the authorities on the disease) roughly half of all cats who test positive (and do not test negative again within a three month period) are persistently infected and show acute signs of FeLV-related diseases. The other half are latently infected and are in all other aspects 'healthy' although they are still carriers. 17% of all cats which test positive (and do not later test negative) will live past 4 years. The next point of view is taken from a much more recent article and has radically different statistics. According to Loar (1993), of cats which test positive for the disease only 5% will immediately become infected with an FeLV-related disease. The other 95% will enter the latent phase which will last for months to years. These 95% are still carriers for the disease and can infect other cats.