How do I know when to consider euthanasia for my cat with leukemia?

I'm very sorry Felix has leukemia! It is a very hard decision to make if you want to euthanized him or not. It really depends on how he is doing at the moment, what symptoms he is showing, and how much you want to invest in treatment. Eighty-five percent of cats who are diagnosed with FeLV die within three years of diagnosis. But regular vet care and good preventative health can help keep cats with this feeling well and can prevent secondary infections. Secondary infections from FeLV can be treated, and cats who develop can cancer receive chemotherapy. If you chose to treat him, you should do twice yearly vet visits with lab testing and parasite control to prevent complications and identify problems quickly. Felix should be kept indoors at all times now. Cats who has bone marrow compromise or widespread lymphoma do have a very poor prognosis. I wish I could give you an easy answer as to what to do, but depending on his current symptoms, Felix does have the possibility of living another couple of years with good vet care and treatment of any secondary infections. It's really up to you how much you want to invest in his treatment. I would be happy to consult with your further on this if you'd like. Simply select the "consult online" button under my answer to talk more.