Average is somewhere around 4. However, I have known several that have lived to see their 5th birthday. I also know of a couple that made it to 6, and one old fellow that made it to 10.
My experience has been that somewhere around 3.5 to 4 cancer shows up and the fight it on. I have had 1 that lived to be 8 (my first), most of mine have passed somewhere between age 4-4.5 of cancer.