Maximum lean angles
If your tire pressures are correct and the tires are in good shape, you can safely lean a Ninja 250 over until the peg feelers start to scrape. It's not a good idea to go much further than that, as the centerstand will drag at really high lean angles and can lever the rear tire right off of the ground. This is bad.
While the 250 won't generate modern levels of cornering force, due to its narrow tires and outdated stock suspension components, it will certainly get through corners faster than many other sportbikes and has no bad habits to speak of. Altogether, you couldn't pick a better bike on which to learn cornering techniques.
It should be noted that scraping a peg feeler, wearing the tires all the way to the edge or dragging your knee are nothing to brag about -- these acts can be accomplished in a parking lot at a few miles per hour. Scraping a peg feeler or scrubbing the edges of a tire are particularly easy if one utilizes poor cornering technique (leaning the bike farther down than your body). Dragging a knee in any but controlled circumstances (an empty parking lot or a track) suggests lack of foresight and intelligence more than it does riding skill, and is extremely dangerous without knee pucks.
A good indicator of cornering skill is being able to take a corner at a good clip without dragging anything. Bonus points if you make it look effortless.
If you want to explore the bike's potential, sign up for a track day -- they're tremendous fun and much safer than exploring the edges of your bike's performance envelope on the street. And remember, track days are about education, not racing.