I'm new at using planes and keeping them sharp. However, you may need to sharpen your irons frequently during your project. Roger's comment about a quick run through your stones frequently is well worth remembering.
If I can feel a burr or I see some wear in the edge I know its time to sharpen. Sometimes it happens way sooner than I would have expected. The bevel angle and the iron material will also have an impact on how long you can hold an edge.
When in doubt, take a few sharpening passes on your finest stone and work your way down if your bade needs it.
That's a great question and one I'm sure all woodworkers think about when using a plane or chisel. First you need to know what sharp feels like so you have a baseline. If you have your planes "scarry sharp" then you know how it should cut and how much pressure you need to use. As you get more experience, you will know after the first few passes if it is sharp. Cuts like butter is the ideal feel. Second, if you are getting tear-out and cutting in the right direction, then time to sharpen. Sometimes grain direction is hard to determine so try both directions.
The other factor is how heavy of a cut you are taking and the wood itself. Adjust the plane for paper thin cuts to start and adjust as needed from there.
Once you have your irons sharp, a quick swipe on the stone will keep them that way. If you wait too long, then your time on the stones go way up so in the long run, sharpening often will save you time and energy.
The title says it all...once a hand plane iron is scary sharp, how often does it need to be honed again to keep its sharpness?
I have a feeling the answer is going to be something like "when it feels dull", but I guess the real question is perhaps to highly experienced hand plane users: Do you sharpen your irons every time you use them?
Example: If I'm making a jewelry box and have a dozen boards of walnut and cherry to plane, am I going to sharpen my irons 3 times during that process?
I'm pretty new to this. In fact, I just restored 3 hand planes to working condition and got them all sharp this weekend.
Thanks for your input!