I don't assume anything. Books say 10-12 years, but anything can happen at any time. I love them every day and cherish every day that they are here with me.
My first ever bun, named Bunny, lived to be about 9 years old. I don't know how we did it; in hindsight I probably didn't give him the attention he deserved. He was very healthy and active and loved Binkying. He didn't really slow down, he just looked older.