How many trouble-free miles can be expected witha stock bike, adult rider, etc...?

All depends on how well the bike is maintained. GSJack got somewhere around 80K on one of his GS's and is currently racking a lot of miles up on another.  On the flip side you can have a bike ragged out at 10K if it is abused.

-Jessie

Treat it well and it will do the same to you 

Treat it mean and it will bite you back

as for km's hmmmm its a tough old engine  how long is a piece of string??

2 miles. Cos right after I get it home, I'll rejet and mod it. Not stock anymore.
Cool.
Buddha.

How long is apiece of string?

You can expect maybe 20 trouble free yards. After that, you change gears. Miss getting it into 2nd. Pause at neutral along the way. Damn bike - not the rider getting used to the bike, NO, it's the bike having Troubles.

Come into the lights too fast and change into 1st. That Clunk is trouble. Bloody bike. Why'd I buy it. Nothing but trouble.

Went wide on a bend but didn't lose it. Bloody bike and poor handling. Must need suspension upgrade. Nothing but trouble.

WHAT!? I've only done 3.5k and need to change the oil. Bloody lot of trouble I'm going to.

Give-us-a-break. Freaking chain lube every 600 miles. More dang trouble.

And what's with the headlight pointing too high/low (take your pick, either way it spells Trouble, not to mention the bulbs that don't last forever, bloody manufacturers)

Keep it lubed, oiled, "serviced" / maintained and the bike will do you well. Treat it like crap and trouble ensues.

Michael

I bought a 97 GS new in 99 and put a little over 80k miles on it and then replaced it with an 02 GS.  One of the exhaust valves on the 97 was down to a minumum thickness shim and would have required some head/valve rework if run a lot further.  My son took it for a commuter bike and ran it up to about 88k miles and then replaced the engine with a low milage used one so no major engine work was ever done on it.

I bought the 02 GS with about 4k miles on it and it had a tight valve bucket that stuck open on one below freezing day and broke an exhaust valve at about 21k miles.  I pulled the head and repaired it and now have 77k miles on it with no further engine problems, just regular maintenance.  I've found that setting the exhaust valve gaps a bit wider stops the exhaust valve regression that the GS's are known for and that my 97 GS had.  I fully expect it to go 100k miles without further problems.  It's almost 40k miles now since I've changed an exhaust valve shim.

Oil consumption on both GSs has followed the same pattern.  No oil added between 2-3k mile changes until 40-50k miles on the bike.  I buy a gallon of 15W-40 heavy duty oil and use 3 qts to change and one for topping up and still go 2-3k miles before the gallon is gone and I change it even with 77k miles on the clock now.