Chain life will depend on your riding style and conditions. I've never used a chain checker but based on what I've read about their accuracy, I could probably eyeball a good chain better than those chain checkers could indicate one. Measuring across the rivets is a much more accurate method. 24 rivets should be exactly 12" on a new chain. If you don't want your new chain to skip, you should replace it before it hits 12 1/16". If you get to 12 1/8", you can be pretty much guaranteed that the new chain will skip on your most used cogs.

FWIW, I recently swapped a 9 speed KMC (not HMC) chain out after 4000 miles of use which measured 12 1/32". I'm keeping the chain and using a new chain which I'll remove at 12 1/32" stretch (hopefully another 4000 miles) at which point I might throw another chain in the rotation or go back to the original. I'll then let it stretch to 12 1/16" at which point I'll install another 12 1/32" chain. Eventually, all of the chains will be stretched past 12 1/16" at which point I'll be ready for a new cassette and chain. In my situation, I'm using a fairly expensive cassette so I'd like to try and get as much use out of it as possible before needing to replace it. The extra effort is well worth it.