Everything in this arc has been building toward the cutting edge. The iron geometry, the cap iron distance, the mouth opening, the skew angle. All of it is managing a cut. And the cut is only as good as the edge making it.
Here's the thing nobody tells you early enough: the edge is a temporary state. Not a property of the iron. A condition you create at the stones and then spend, pass by pass, until it's gone.
How fast it goes depends on what you're cutting. A Bigleaf Maple panel, loaded with silica, can take a fresh edge from crisp to struggling in forty minutes of smoothing. Western Red Cedar might give you all afternoon on the same edge. Domestic hardwoods fall somewhere in between. The edge is always running down. The only question is whether you notice before the wood starts telling you.
Three signals, in order
The shavings change first. A sharp iron pulls continuous, translucent ribbons. As the edge goes, the ribbons get thicker, the edges more ragged, and eventually they start breaking into short segments. This happens before you feel anything different in your hands. If you're watching the shavings come off the mouth (which Post 93 told you to do), you'll see the edge failing before you feel it.
The resistance increases second. A sharp plane feels like it's being pulled into the cut. A dull one feels like you're shoving. You start leaning harder. Your arms engage more. That's not the wood getting harder. That's the iron.
The surface changes last. A sharp iron leaves wood bright under raking light. A dull iron compresses fibers instead of severing them, and the surface goes cloudy. Run your finger across a just-planed face and it doesn't feel clean. Check the iron first.
The real problem is "one more pass" thinking. The iron produces worse work before you consciously register the change. You come back with raking light and find a burnished patch, or raised grain that absorbed the cut instead of parting with it. The re-sanding costs more time than the resharpening would have.
The thumbnail test
Press the back of your thumbnail against the cutting edge and drag perpendicular. A sharp edge catches. A dull edge skids. Thirty seconds from plane to answer.
Before going to the stones, try the strop. Five strokes on loaded leather can realign the wire edge, the micro-curl at the tip that forms during use. Stropping doesn't remove metal. It folds the edge back into alignment. If the thumbnail test goes from skid to catch after stropping, you just bought yourself more time. If it doesn't, you go to the stones.
Knowing which problem you have determines whether the fix takes sixty seconds or five minutes. That distinction matters when you're three hours into a smoothing session.
When to stop mid-pass
The instinct is to finish whatever stroke you started. Mostly that's right. Stopping mid-surface leaves a subtle transition line, and on a show face you don't want to read that under finish.
But if you're getting real tearout, not the adjustable kind but the iron-is-done kind where fibers are lifting and compressing, stop immediately. Six more inches of a dull iron pressing fibers down does more damage than the short track of a stopped cut. Fix the iron. Come back to the surface from the end.
That's the exception. Most of the time: finish the pass, then resharpen.
Three ways to get the timing wrong
You sharpen too often. Every plane makes shavings for a while. If you're at the stones every fifteen minutes on domestic hardwood, the problem isn't the edge. It's the geometry behind it. Probably an included angle that's too acute, or a secondary bevel collapsing fast. You need more steel behind the edge, not more trips to the stones.
You push the iron past useful work because resharpening feels like an interruption. It isn't. The sharpening is part of the session. The break at the stones is where you recalibrate, recenter, and come back with a tool that's actually cutting. The woodworkers who move through a session without apparent friction have figured out that sharpening and planing are one continuous process.
You sharpen on a schedule instead of reading the iron. An hour passes, time to sharpen. That's not wrong, but it's not paying attention either. Some sessions eat an edge faster. Some species, some grain orientations, some cutting angles are harder on the steel. Let the shavings and the resistance tell you when. The clock is a backstop, not a trigger.
The rule
The edge is the constraint everything else serves. Take care of it at the beginning. Take care of it when it tells you to. Five minutes at the stones is not a cost. It's the reason the next forty minutes work.