I'm of two minds about the 'scrape your nail' method of determining your sharpening.
First off, people I respect, that are more knowledgeable that me, with many more years of experience, use it. These are people who are professional skaters with thousands of hours on the ice.
But a good skater, with good technique can skate on a sharpening that's close to 'dead' because they do have good technique. To them, the 'scrape your nail' method may be adequate.
I don't have good technique. I like a sharpening when I start to skid on elements I'm good at. The blades can still draw a curl on the nail, and my sharpener rolls his eyes a bit when I ask for a touchup.
I'm an adult. I can do what I want. If I like my blades sharpened sooner than my coach feels is necessary, it's my money.
I think I've gone as long as 60 hours skating 5 hours a week between sharpenings, so I'm not particularly hard on my blades (no jumping or spins). I've never had them sharpened when they couldn't draw a curl, it's too hard to schedule. I usually have them sharpened when I skid, or I have a big dent in the edge.