My Mrs can now answer this question properly, having just survived 3500 miles over two weeks.
We had the OE seat comfied up a bit by Tony Archer so it has an extra inch of foam to assist, and a bit of re-shaping.
For the really long motorway days she still uses a stadium seat, a cheap way to ensure all day long comfort, but for the rest of the body we both found it very good, and the pillion is high enough to easily dangle legs if needed.
Every time we stopped for a stretch / arse rest, it was on my request, she seemed to be able to go pretty much all day long with nothing more than the odd leg dangle.
Solo I can go much further as I can put my feet up on the crash bars, or stand up, but normally my long legs cramp a little on a long run - again I can dangle them a bit on straight roads, but my arse still gives out after a couple of hours - but I have always struggled to sit still for long periods even in the car.
Overall the bike left us both fairly fresh after long days, and I never felt like I could not have ridden a bit further even after 12 hours on the road.