My experience is that I can go for about 4 hours without problems and I find the seats in the Morgan very comfortable. The ladies usually require a pit stop long before I start to feel uncomfortable so the comfort factor is not what dictates stopping.
The usual thing is that just as you are ready to go they say "I'll just duck in to the loo before we leave". Ten minutes after your planned departure time you get away then after about half an hour or so it's "Keep an eye open for a service station /bar/Cafe please". After that you might manage to get in a couple of hours without a stop and if you are lucky you can stretch it to lunch time