How long will a Ninja 250 engine last?
The Ninja 250 motor has a typical lifetime of about 50-60,000 miles. This is highly variable. One rider (Bill Hoddinott) had a motor last 88,000 miles. That motor was taken out by a failing camchain tensioner; Bill reported that the rest of the motor looked fine upon tearing it down. Some have reported needing replacement before 50,000, although this is infrequent and usually due to abuse or severe neglect.
The engines in the late-80's/early 90's EX250s had pretty soft valve seats. It was not uncommon to run out of valve clearance around 30,000 miles. If you have one of these, check the valve clearance often.
If your engine has developed a problem large enough that you're considering rebuilding it, also consider buying a used motor and installing that instead. Used motors with reasonably low mileage are available on Ebay, in our classifieds, and from motorcycle salvage dealers. A typical example costs about $500 plus shipping. They can be found cheaper, depending on condition and the seller. Installing such a motor will most likely be cheaper and involve much less labor than rebuilding your motor.
If you're concerned that this high-mileage bike you found is about to blow its engine, don't worry. Unless it's got a lot of miles and has been poorly maintained, it'll be fine. A used bike's mileage is secondary to its maintenance and appearance (in the sense of whether it looks abused or not). An abused low-mileage bike will probably not last as long as a reasonably maintained higher-mileage bike.
If you want to keep your bike running for as long as possible, stick closely to the maintenance schedule recommended in the Owner's Manual.
Things which will reduce engine life, in order from most to least devastating:
Other than these things, the Ninja 250 motor is pretty bulletproof.