I think the average once in a lifetime buck is probably around 140-ish. In some places that's a monster, in other places that's average.
But I define it like this: It's all about expectations. In all the years I hopefully (Lord willing) have left to hunt, do I expect to see a 140 class buck? Absolutely. 150 class buck? Probably. 160? Probably not. 170? No.
Fortunately, I already have that once in a lifetime buck, netted 173 4/8. He's the biggest thing I have ever seen, and I never ever expect to see a deer like that again. But you can bet I'll be looking.