I did a 2hr+ ride in Southern California and for the most part I had no trouble keeping up with the flow of traffic at around 75 mph on the 5 freeway. At 75-80 mph you are decently high in the RPM range (9k?) in 6th, it's not annoying or bothersome, but like iVisionX01 said, it's the wind that can be quite annoying.
I did a 2hr on all 2 lane highway. It wasn't bad at all I wish I had done it more often now that winter is coming around. Like everyone said I was at 6th gear around the 5k RPM (60mph) and 7k RPM (70mph) most of the time. I did fuel up on gas every time my gauge said I reached half just to be on the safe side, I do that with my car and just about anything that runs on fuel. I drove on the right lane all the way except passing Semi and slow old people. Great ride though the wind wasn't bad Id say it was a great day to ride too, but for a bike that light I could see it being an issue on winder days.
Welp ... I'd say that 2+ hours of freeway cruising on a motorcycle is a waste of good tires. Still, I've done a fair amount of commuting on mine, meaning 30-40 minutes of freeway at a time (at least in the outbound direction, when I have to be somewhere on time). That'll typically be right around 8K RPM, and at that range, the R3 is one of the least buzzy bikes I've ever ridden. It's hard to believe it's a twin, even. Illustration: while riding the R3, my activity tracker will give me idle alerts as though I were just sitting at a desk.