I think this is one of those questions that could get a thousand different answers to, like tires and oil. There are so many variables, and most have been hit on already. Your bike, age, experience, physical condition, amount you ride (seasonal, year-round), how you ride, desire...it all plays a factor.
I don't expect either one of us to get on the bike in March or April (whenever the snow melts
) after not riding since November and put on 300 miles the first day. You have to get your body adjusted to it. However, once it gets to May or June, we can put on 300 miles in a day without a thought, although we like to stop a lot. Personal preference plays a factor in comfort, too. Our 1100 Silverado has stock seats, which some people think are horrible, but we find ours comfortable (I think it's more comfy than the fancy computer chair I'm sitting in right now).
I personally would have no problem riding from Wisconsin to visit our daughter in North Carolina or my folks in south Texas (both of which I've suggested), but my wife isn't too sure of that idea. I've been riding for 25 years, she's been riding for 5, so I'm sure that's another factor.
It's hard to really put a number on a question like that. Get on and ride, and when the tank's empty, fill it up and ride some more or head home and ride again another day. Have fun and ride safe, that's what it's all about