Sharpen when it stops cutting, not on a calendar; for most daily users that is a strop every few days and a proper sharpening every few months. To check an edge, Böker USA calls resting it with its own weight, loosely on a thumbnail, a reliable and low-wear test, and says the cutting test in paper should be avoided because the blade can be severely damaged by the paper.
Böker USA says the cutting test in paper should be avoided because the blade can be severely damaged by the paper, and calls resting the edge with its own weight, loosely on the thumbnail, a reliable and low-wear test for sharpness. Buck's how-to page runs the knife over a fingernail lightly, and stresses lightly, to feel whether its sharpening has reached the edge.
Either check is more reliable than looking at the edge, because a dull edge and a sharp one look identical from a foot away.
Most of what people call dullness is a rolled apex, which a strop straightens. Do that regularly and full sharpening becomes an occasional job.
See honing versus sharpening for what each one actually does.
What you cut, far more than what steel you own. Cardboard and dirty material dull everything fast; food and cordage are gentle. Cutting on glass or stone boards destroys edges regardless of alloy.