How long should a well maintained chain last and what can I do to prolong its life?
A new chain has factory installed lube inside on the bushings and pins. It is a thicker, stickier lube than anything you can put back on the chain after a cleaning which is that thorough. 

I love that you take time to maintain the bike. Definitely check your chain checker, as Daniel said, but on your next chain, try relaxing the maintenance a bit. 

Clean the chain by spraying a light citrus degreaser on a rag, folding the rag in the palm of your hand, and running the chain through the rag. This will clean the exterior and the accessible bits without soaking away the internal factory lube. 

Once clean, lube the exterior of your chain with a quality lube (I like Boeshield T-9 or Dumonde Tech yellow) and wait 15-20 minutes. Then wipe off all excess lube. 

Starting with your new chain, do that for the first 6 months, or until your fresh lube job sounds unlubed again within a few on the bike hours. 

Once that is not working, go back to your current method. I'd say do the complete strip and relube once a month, with the process described above on the in between weeks. 

After your next chain, let us know if and how your mileage changed.