Are we talking a thin edge or a sharp edge? Mine are sharp enough to slice paper without grabbing, it's what I consider a good utility sharp -- don't need to shave with it. I do not thin the edge or otherwise change the blade profile.

hanging hair test

It depends on what I use it for.

When I used to cut a lot of rope, I had a blade that wouldn't cleanly slice through a sheet of paper but would put the hurting on some braided rope. I considered it more then adequately sharp for the job.

The knives I use for splitting bones on wild game are generally the same way. Don't know a way to test them other then rip a few rib cages with em and see what edges work best.

The knives I use for skinning are kept as sharp as possible without stropping.

My general utility/EDC knives are kept just sharp enough to do all my daily tasks without extra effort required by dull blades. As soon as I notice they are getting to the point that they -need- sharpened (usually a few nicks in the blade, and generally just dull) I'll sharpen them up enough to slice paper and let them be for another 4 to 6 months. Rinse repeat.

The only knives I obsess over are my kitchen knives. I don't make them razor sharp, but they receive a touch up on a -very- regular interval.

There is no upper end on sharpness. Sharper is always better. A knapped edge on glass or obsidian is many times sharper than any conventional metal blade and they aren't perfect. But there are practical limits for how much time and effort I'm willing to dedicate to getting a sharper edge.

If a knife edge won't skate on my thumbnail, it digs in, it is sharp enough for most uses.

Materials used for knife blades are a compromise between, toughness, flexibility and shock resistance and the ability to be sharpened to an edge and hold that edge. Most common alloys used for knives won't hold a very fine edge in practical use. Those alloys, or materials, like the ceramics, that are hard enough to hold a very fine edge are often so brittle that the blades can fail catastrophically in use. Some are so brittle they can snap if dropped. Extreme cold can induce similar brittleness in some otherwise practical alloys.

I hold the blade out, cutting edge toward the sky and then toss a silk scarf up in the air and let it slice in two as it falls in slow motion to the floor... oh wait... that was a dumb a$$ movie my girlfriend made me watch.

It's one thing to cut WITH the grain of wood fibers (the old whittled stick approach), but the old woodcarver's used to test a knife's sharpness by trying to cut cleanly ACROSS the end grain on a stick of soft pine. If a knife can do that without digging in, chattering, crushing or popping off little chunks of wood, and leave a clean polished surface in the blade's wake, then that knife is SHARP. It is very satisfying to get a knife this sharp because the blade makes a pleasant "whisking" sound as it slices directly across the wood fibers. It also puts a pleasant piney scent in the air.

The late Donald Crabtree was the dean of american knapping with obsidian. his french counterpart who worked flint couldn't even work with obsidian without considerable practice. The stuff will literally flake to one molecule thick ( I know, I saw one under an electron microscope.)
Crabtree in fact made up a surgical kit for his heart surgeon. the doctor had to practise using the tools, retraining his hand muscles to the greater cutting ability. Crabtree underwent heart surgery, and the finer incisions healed so rapidly glass instruments are now common.
That is sharp. But the advantages of metal gives us different standards. I don't make many fuzz sticks in the wild places out of paper. Testing the knife on what you actually do, like our member butchering game is the best guide.
In the real world, there is sharp,and there is sharp.
An apocryphal story is worth remembering. During the Crusades, King Richard met Saladin ( in reality, they never met.)
Richard drew his great sword and with a thunderous shout cut an anvil in half.Saladin threw a silk sash into the air, turned his scimitar blade upwards and watched the falling silk fall in two pieces.
Meanwhile, I believe several ancestors of contemporary knifemakers were stabbing castle walls.

That story is not just apocryphal, there are two other major fallacies. First, no sword could cut an anvil in half (especially not a medieval European one - plenty of research has been done in that department recently). And second, Saladin couldn't have had a scimitar because it simply did not exist as such at the time. In Egypt and Syria straight swords were still predominant at the time. The saber became popular only a couple of centuries later and the scimitar was a more recent development still.

If the story is that inaccurate I wonder if the silk scarf cutting test makes any sense at all?!

I think it depends on the use. An old style shaving razor might cut through thick rope no problem, the first time and be useless after that.

I have blades with VG-10 and S30V steel and while VG-10 can get noticeably sharper it also gets dull quicker. That must be why Doug prefers S30V in the field.

My sharpest kitchen knives might have a problem with the skin of a tomato but my knives with a a bit rougher edge manage to cut through no problem.

I lightly drag the blade across my thumb - maybe at a 45 degree angle - and if it catches pretty good on the friction ridges then I feel it sharp enough for what I need.

I sharpen my knives using a Spyderco Sharpmaker which does a great job pretty easily. If you're using a Sharpmaker on one of Doug's Mkx knives, don't forget to use the 30 degree holes to maintain the original angle.