How long can it reasonably last? Depends on how reasonably it is maintained and ridden. Modern bikes have things like oil coolers and filters and liquid cooling and electronic ignition and fuel injection which make them MUCH less likely to overheat, score pistons, wear out crank bearings, etc. I'm in a group and forums because I have a bike like that (Suzuki Vstrom), and many folks report 50 to 80,000 without anything other than oil changes, fresh plugs, tires and brake pads.

On the other hand, I sold a Triumph to somebody and it only lasted about 40 miles or so (don't let stupid people play with a bike if they don't know what they're doing - "friend" sat on the bike revving it, popped into gear, went into the back of a car 15 feet away. Engine, trans, etc, trashed, "friend" walked away).

You might like this forum for bike info https://www.advrider.com/

The CP2 engine used in the Yamaha XSR700 has been around for a long time, first in the MT-07 and later the Ténéré 700 and is well-known for reliability. Despite being priced at the budget end of the spectrum, the build quality is excellent and you should have no problems from a well serviced used model.

https://adventure-motorcycling.com/review-yamaha-xsr-700-5000-mile-test/

I had my 1979 Honda CB750K for 33 years before I sold it.  I was meticulous with the maintenance.