Here's some more information for you, Allie. An FIV positive cat could live many years without any symptoms. I am trying to find an exact percentage of times the full blown virus manifests itself. So far, the information is more general.
Feline immunodeficiency virus (FIV)
• FIV behaves similarly in a cat's body to the way HIV (AIDS) behaves in a human body. FIV is not contagious to humans or other species.
• FIV is contagious to other cats, but difficult to transmit - FIV is transmitted primarily through deep bite wounds resulting from fighting or rough play.
• A positive screening FIV test does not necessarily mean a cat has FIV. A Western Blot test or IFA test should be conducted. Re-testing and the timing of re-testing should be discussed with your vet. Note that positive FIV test results for kittens under 6 months of age are not considered valid - such kittens should be re-tested each month using the SNAP test until they test negative or exceed 6 months of age. If a kitten still has a positive SNAP test result at 6 months of age, it would be appropriate to run a confirmatory Western Blot test.
• The good news about FIV is that symptoms may never appear or may not appear for years. That means many FIV-positive cats can have a normal life span.
• Providing a home to a healthy FIV-positive cat is just like owning a healthy non-positive cat, with the exception of being extra sensitive to symptoms of illness. The cat simply stays strictly indoors and preferably has no contact with non-positive cats - although introducing another FIV-positive cat would be fine. A good diet, lots of love and attention - and the cat's all set. Remember, the primary means of transmission are through bite wounds resulting from fighting and rough play. Casual spread of FIV does occur, although it is uncommon.
Important note on test results for both FeLV and FIV: A positive test result may be a false positive, for example, due to a faulty test kit or a test administration problem.
This is why re-testing is so important when the first test is positive. Also, any cat given the new FIV vaccine, Fel-O-Vax FIV, will test positive for FIV.
Here's a snippet from another source:
Cat-to-cat transmission of FIV in multiple-cat households where there is no fighting among cats appears to be quite uncommon. Many FIV-positive cats are not diagnosed until after they have lived in the multicat environment for years. Ideally, any infected cats in such households should be separated from the non-infected ones, but in reality, if fighting or rough play is not taking place, the risk to the non-infected cats appears to be low.