This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
There is no set time period for how long an FeLV+ cat will live. One person on the internet said they had a cat which lived for 20 years with the virus, while others have given dates as long as 10 or 12 years, although these are probably extremes. I have not found any truly long-term studies to document, but it seems that of the studies done, 83% of FeLV+ cats do not live beyond 4 years. (Hardy, et al 1980). All cats which do not later test negative, but in all other ways are healthy, are carriers for the virus.
Even though they do not have acute symptoms, they can still spread the virus to cats which are not infected. Often people do not have their cat tested for the presence of FeLV until the cat is noticeably sick, and by this time the FeLV-related disease may have progressed too far for the cat to recover.