He's lasted roughly as long as Swayze did with metastasized pancreatic cancer so far. I like Sam, he's probably in large parts solely responsible for The Simpsons surviving beyond its rough first series, but Matt Groening had it sussed, I think, when he said Simon was "brilliantly funny and smart, but unpleasant and mentally unbalanced".
Or to put it another way, he can be a sod all he wants, he created Mr Burns, and Burns will last longer than he does.