How long is the life of an average new motorcycle?

That all depends on how well and how often it's maintained. I would expect well over 100,000 miles.

You need to be a little more specific.Some little Chinese piece of crap will not last as long as a Gold wing or a BMW. Some bikes are dead and not worth resurrecting after a season or two. I`ve pulled more than 250,thousand miles from a Gold wing (1982),and still ride an 80 inch shovelhead Harley that has way over 200,000 miles on it. My GF has a 1972 Honda cb 500. Get a decent bike and then maintain it,you should be fine.

You would get better answers if you narrowed the herd down to a few specific bikes that interest you?

Your question is too broad to realize anything close to a practical answer. Size of engine, class, and brands would help greatly.

For instance a Goldwing or Moto Guzzi are proven engine designs that outclass durability in engine life by a wide margin over other bikes. These are just two examples of many upon many.....

The Honda little wing is proving to be a mile maker as well.......

What interest you would help answer your question

There are a bajillion variables, the type of bike, maitence, and riding style among the top of them. Sportbikes are typically destroyed (theft crash etc) long before they wear out.

there is no reason a well maintained road legal motorcycle can't go 100,000 miles easily.