A vet friend of mine says she personally would vaccinate an indoor cat annually, for the first two years of life, then every 3 years after that. But, because the vaccine manufacturers still recommend vaccination annually, that is what she tells her clients. She said if she told somebody to vaccinate every 2 years, and after 1.5 years, the cat got sick, and the owner complained...she could lose her vet license. So, off the record, she is all for every 3 years, but she won't officially say that. I can't say I blame her.
My own vet told me to vaccinate the cats for FeLV for the first two years of life, then not to worry.
I still think it's important to have your cat checked over by a vet every year though.
There's some vaccine related links here...