It really depends on how well your bike fits you. The Monster fits me really well, and I have found that 500 to 600 miles in a day is pretty easy. 700 takes some planning and intent; 900 is the most I've done in a day, and that was a long day. (Although I might just go for a Baby Iron Butt and do a certified 1000 mile day this year, just to be able to say I did.)
I have a good friend with a Monster 620, and she travels and goes camping and so on on it. She finds 500 miles in a day to be about her limit for comfort, although she has done more.
The first thing I would say is to work up to it. Don't run out with your buddies and try to keep up on those long days from scratch. There are muscles you will need to build up (especialy, on an unfaired bike, your neck muscles). I would do a bunch of 200 or 300 mile days first, and see how it goes for you.
On the topic of neck muscles, speed makes a difference, so it also depends on how fast the people you would be traveling with ride. For me, once I hit about 100mph, it really is a strain to keep the helmet and head up and stable, and I can't do that for a long time. So if I'm doing more than a couple hundred miles in a day, I have to keep it below 95mph except for short bursts like passing a garbage truck or something I really don't want to be behind.
The only modifications I have made to my bike for this are (a) a Crampbuster ($12 and totally worth it), and (b) I geared it down a bit -- went from a 39 rear sprocket stock to a 37 (which is the opposite of how most people regear their Monsters; they usually go shorter not taller).
With the colder weather here, I have bought a pair of dirtbike handguards, which I plan to fit to help protecft my hands from the wind on colder days. It's pretty much stopped snowing here, which was my limit for riding, but it's still cold. I started riding again last week, and the cold does show up in the hands first.
I found the original seat to be pretty good. When it wore out, I got a Corbin, which I was not really happy with. Whan that wore out, I got the original seat re-covered, and that's what I've got again now. The SD to Cleveland trip did leave me a little saddlesore -- I only had 4 days between jobs, so that was one 900 mile day, followed by three 600 mile days in a row. So if I was to take up touring seriously, I probably would research to find a better seat.
Do you mind if I make this into a thread? I think it would be interesting to get more people's input on touring on Monsters.
1993 Ducati M900 Monster "Patina" (265,000 miles, killed by minivan 30Oct17) -- 1995 Ducati M900 (wife's bike) -- 1979 Vespa P200 (daughter's scoot) -- 1964 Vespa GS160 (160cc 2-stroke) -- 1962 Maicoletta scooter (275cc 2-stroke) -- 1960 Heinkel Tourist 103A1 scooter "Elroy" (175cc 4-stroke)