Probably around 1100. Multiple high-mileage days and very high temps -- on a CC trip in August 2008, I left Omaha and had record heat all the way into Billings (via Cheyenne, to avoid Sturgis), where it was 98* upon arrival at 10 pm -- add to the need for a long recovery when the trip is over.
I was feeling good on my way home from Daytona one year and punched out a bit under 350 miles without putting my feet down (on an 1100GS with TT tank), a memorable leg for me for its ease. Every now and then the wind will be at your back.
A well-setup bike really helps -- legroom, wind management and cruise control have become more important to me over the years. A CamelBack also helps, as does (surprisingly) a well-tweaked suspension. I don't listen to music.
Surprised that no one has mentioned John Ryan