What is the average life of a motorcycle chain?  Is 12k miles normal?

Don't know.  I wait till it breaks.

Probably. Keep lubed.  Adjust tension, then pull on the chain at far end of wheel sprocket.. Should move off  no more that 1/2 link or it's worn. Always replace a worn chain, if any doubt. Cheaper than later replacing sprockets...

Depends on the bike, how it's ridden, the chain quality & maintained.

I used to get 1 to 5 thousand miles on a Honda CB350. The low figure was a cheap Chinese bit of junk. This was pre O ring chains. Normal procedure was to oil it weekly + after riding in the rain.

Use, maintenance and type of bike will make a difference.

12k miles is not impossible.

Depends on the type of chain and how often cleaned and lubed. Butcha don't tell us that.

Chains stretch as they are used. The way to decide if the chain need replacing is to look a the sprockets they drive. If the drive gear sprocket is egged shaped replace the motor sprocket, the wheel sprocket and the chain.

20-30k for o-ring type chains if kept cleaned and lubed properly.