Well, I've got a bike that I've nicknamed "Couchasaki". I have a big wide mustang touring seat with a backrest, highway pegs, nice comfy grips, I generally wear gel-lined gloves. I've got a stereo, mounted to the handlebars, and a tall windshield. I can go about 250-300 miles on a tank of fuel. By then I'm pretty sore, so about then (250 miles or so) I'll stop, get gas, drink lots of water, and rest for 5-10 minutes. After that, I may be stopping ever 75-100 miles or so to drink some water to stay hydrated and get the blood flowing to the rear-end.
I do a lot of long distance riding though and I've found that when you do a lot, you start to firm up the right muscles and you can do it longer. On a nice day, it's not at all unusual for me to just hop on the bike and ride 100-150 miles or so, just riding around the countryside (I live in a rural area), ride through a couple unique small towns and watch the people as I cruise slowly through town, and then get back home without ever getting off the bike. When I first started riding, maybe 50-60 miles and I was sore and wanted to get off. Now, I'm about as sore after 250-300 miles as I used to be after 30-40 miles. Part of that is the aftermarket seat and my highway pegs, but a lot of it is building it up (I can ride farther now than I could a year ago with the same accessories).
So moral of the story, get out there and ride! Take those long trips every now and then!