I chose 15 minutes or less, but really, it does depend on the situation. And the kid. With the younger one, who is still almost exclusively breastfed (we've started with little bits of solid food), it's not as urgent to get it done right away. If we're on the way somewhere that starts at a certain time, like church, I would wait until we go there. With the older one, if I don't change her within ten minutes, she gets a bleeding rash. But she almost never gets a dirty diaper in the car. I think the last time was over six months ago. So I didn't factor her in, but if I had, I would have said "next exit."
ETA: On public transportation, no way would I change them out where everyone could see. I'm not real familiar with trains, so I can't answer for that, but I wouldn't change them on a plane or bus. My older daughter is very tall, so if we were in the middle of a long flight, and the bathroom was too small, I don't know what I'd do. Changing her standing up is still not possible; she's still a little unsteady.