Welcome, Jen! It's hard to answer that question, because some cats are diagnosed with life-threatening complications, and in other cats the diabetes is secondary to some other problem.
But for an uncomplicated case where the cat "just" has diabetes, and the owner takes precautions to make sure that the complications of hypoglycemia and diabetic ketoacidosis are unlikely, it's not at all uncommon for the cat to live for years after diagnosis. I think that 14 years may be the record on this board (Jojo's Bunny, who was diagnosed fairly young).
Please tell us more about Sam, so that we can help him join the Longevity Club!
-- Janet