I regularly ride mine on NJ and NYC highways (NJ Turnpike, I-95, Garden State Parkway, Henry Hudson Highway etc). I typically ride 55-65, faster in short bursts for passing. vibration and wind above 65 is uncomfortable for me. Needless to say, at such speeds, I'm one of the slower ones on the highway.
I've also taken longer trips (200 to 400 miles one way) but i take these in 2-3 hour stretches with a long rest in between. For example, for one weekend trip to Letchworth State Park and Watkins Glen (great destinations by the way), I rode around 2 hours (starting from Jersey City area) on Friday night, rested for the night, and continued the rest of the way for another two hours the next morning. I try to avoid riding more than 3 hours in one stretch because by then I'm already feeling some fatigue.