It depends on how much it's played. A Steinway in your home is likely to last a lifetime or more. The Steinway in the recital hall at our school is between 20 and 30 years old (I think), and well past its prime. It's played (hard) probably 4-6 hours a day, and eventually they just wear out. It would make a fine piano for someone's house, but it's no longer a performance-quality instrument. (Our own personal piano is a pre-WWII instrument, a 5'9" Knabe with ivory keys. It's been rebuilt once, and is a little...quirky, and could probably do with another rebuilding. It was also a very good deal for two just-out-of-grad-school musicians.)

Some music schools have contracts with Steinway where they get all-new instruments every year, which are sold back to Steinway at the end of the year who sells them as used instruments. I'd imagine (but have no evidence) that music schools in general are one of the largest consumers of pianos. At our undergrad institution of ~200 majors, we have probably 30-40 pianos in the building. My graduate school probably had upwards of 200, and employed 3 full-time piano technicians to maintain them.