I don't know if there's a golden number, but I plan on spraying the piss out of my 421 this season. To the MOON, baby!
I have my own torque plates, which I think is a huge benefit when it comes to quality machining. Good hardware. Light rotating assemby. Correct ring gap and plugs range. And I have Ross add extra material around on the crown of my pistons. I also build my nitrous motors loose.
When it comes to carbureted nitrous motors, detonation is the killer. Because ultimately, it's impossible for a carb to distribute fuel equally to each cylinder. So eventually, the carb/intake is going to pick on a cylinder, and if you're not careful you'll window your block. On the big SHOT nitrous motors (300+), I much prefer EFI because each cylinder has its own dedicated fuel source and it's more controllable.
But if you have a quality motor with good parts, quality machining, adequate fueling, and you follow the golden rules, it should live for a while. If you f-up, it'll bite you. That's the game