I've only been riding for just under two years. I have maybe 6,000 miles under my belt. I've taken 3 motorcycle training courses (I don't think just taking the DMV test is enough, and I don't care how long you've been riding. That just means you probably have been practicing errors over and over again).
My father who use to ride for years (who hasn't taken a course) make me cringe on some of his riding choices (riding in the center lane almost all the time or mounting the bike from the wrong side). My friend who has been riding for 10x longer than me made the worse riding error I've seen--I've gone far enough to tell him to tell him about it: NO rider should be following close to large vehicles through an intersection without allowing yourself to SEE AND BE SEEN by the traffic ahead. Stay at the stop light and allow the vehicle to proceed to give yourself space to be seen. See and be seen is the basic fundamental rule in motorcycling that some people don't get.
The reason I stop often isn't so much physical. I'm 6'4, 220 lbs, and 12% body fat. If you do the math, this equates to someone with a lot of muscle mass...I get mistaken for being in the army.
The reason I stop is because I've learned to RIDE TO SURVIVE. I'm constantly looking for animals, sand, and cars. Every second. Rarely ever do I "relax" on a motorcycle....and those periods are pretty brief. Zero close calls so far, and the situations that I could have gotten hurt from I saw 20 seconds in advance and laughed at them in when they arrived. I must be doing something right. Don't mistake my confidence for complacency, I always ride like I'm invisible.
How often also depends on the road I'm on. There's lots of twisty, sandy, and traffic-filled roads here. Idaho is known for being a serious cycling state. We also have the highest motorcycle injury or death rate in the nation. I'm usually riding my girlfriend around on short trips, which does drain a little more. When you're cruisin' without a windscreen at 75, you get more mentally fatigued too. Plus as I've revealed, I don't have a whole lot of experience, so I suppose my "50 mile limit" will get better and better. I know that if I got on the interstate right now (I have VERY little experience on interstates), I bet I can rack up 100 miles without wanting to pull over. The type of road and driving situation counts too is what I'm saying.
So yes, I stop often for mental reasons. To give my mind a break so I can be PREPARED when I get back on, and to take a 5 minute look at a lake, mountains, or nature. It's nice to take a stop to enjoy things that you just pass by...
btw, my ass is never sore on my '77 CB750 seat. The CB750 seats are pretty comfy actually. Try sitting on a Harley seat. I rode a Harley person home when her bike died, and she went on and on about how much my seat is like a couch.
The advice that should be given to a person who doesn't have a lot of riding experience is to take breaks when you feel fatigued. I don't care how many miles it is. In case you didn't know, motorcycling is about the journey and not the destination.