What is the average lifespan of a ping-pong ball?

Average life span of a ball could be anywhere from 2 seconds to years. maybe..... No matter the type of ball, if you accidentally step on it, it will dent, and that's usually the end of THAT. *sometimes you can use steam to pop out the minor dents*
Table Tennis balls are generally graded from 1-3 stars. 1 star balls are the easy populous general balls, intended for rec use only. trust me.1 star balls do not bounce as well or consistently and tend to have weak spots and fail easier, though they ARE cheap. 2 star balls are slightly harder, less imperfections, some can be used for national play, 2 star balls are used in Robots, like my Newgy 540, serve drills, multi-ball practice, etc. 3 star balls, depending on the makers, range from national to international play quality. They bounce the most consistently, and have very very very few weak spots, imperfections and take much longer to fail. Nittaku is the premium maker for table tennis balls, their Premium 40 mm 3 star balls are regarded as the best in the world. Other notable makers are Butterfly, 729, and Halex.
www.butterflyonline.com -Butterfly table tennis products
www.zeropong.com -Chinese Table Tennis equipment, from 729, LKT and others.
so there you go.

It all depends on the ball and how good of a player you are. If you spike it or hit hard a lot it may only last a day. I'm pretty sure that the type of ball is bad quality. So if you don't have a good type of ping pong ball it may not last long at all !!!

There is a life span for a ping-pong ball, but I have never waited till the life span that ball. Because usually we play with that ball immediately after the purchase of the ball and the normal ball will last for at least for 20 to 30 days if you are a regular player. Otherwise that will long last for more than a year.