For me it depends on how well by butt is rested from the last ride. If I give it two days off, I can go 20 miles without even thinking about standing up. There are some days I need to stand every 20 minutes. This is with a Brooks B17.
I've also noticed that doing the right stretches before I start can help keep me in the saddle longer. I'm not talking about anything intense, just flexors and groin stretches.
Another factor is your pants. You get what you pay for. I'll probably be throwing away two pairs of bargain shorts I bought on clearance.