FeLV is an immune disease. As such it isn't actually the FeLV virus that is the problem but that it suppresses normal immune system functioning making the cat more succeptible to other illnesses. So symtomatically, it can be just about any type of symptoms of any other infections or diseases. The younger the cat, the bleaker the prognosis since younger cats have less developed immune systems. If you have other cats you need to have them tested as well.
One of Swampy's cats died of Leukemia -- this is before there was a vaccine available.
He was very sick and in a great deal of pain -- he was very lethargic and would cry cry cry. One night he never woke. He lasted about 6 months from diagnosis.
Sorry, but with Swampy's personal experience, it is hard for him to give you an optimistic word in this situation.
Oh sweetheart, I'm so sorry your kitty has Leukemia. God bless you for taking care of a stray cat with Leukemia. That is so kind of you.
You will know when the time has come. Trust me. They have their ways of letting us know. Watch for signs such as lack of appetite, not drinking water, lethargic and not wanting to play, hiding, sleeping too much, nauseous, not able to walk very well, weight loss and dehydration.
I have never had a cat with leukemia but am sure it is devastating.
Good Luck