I get some discomfort around oh, 70 or 80 kms or so, but it does depend on which bike I'm riding and how I'm riding.
I did a very fast (for me) 100k group ride recently on my Scott and I don't recall any issue with my butt getting sore, but that was an aggressive ride and I wasn't likely putting much weight on the saddle for most of that ride
Conversely I completed a tour recently (
International Selkirk Loop) with a small group of friends on my Diverge, and after the first day my butt was killing me and I was in agony for the rest of that ride. The last day I must have stood on the pedals for half the ride!
That turned out to be an issue with the stock saddle more than anything else. It wasn't something I'd noticed before but that was the first time I've ridden that bike for those distances day after day.
After the tour I swapped the saddle out for a Specialized Romin that I had on hand and my comfort level improved dramatically, even on a 90 km ride I did shortly after the tour.
But even on my AWOL with a B17 I will usually get some discomfort around the 80 km mark. It's not normally the kind of agony I had on the Diverge before swapping the saddle though - I don't ever want that kind of discomfort again!