Regarding how often to sharpen blades, the number of skating hours I've been told by various skater techs a range from about 20 to 40 hours. Personally, I goi about 24-ish hours between sharpenings, which works out to about every 5-6 weeks. I try to keep a regular sharpening schedule just so it's easier to remember, unless there's a week I skate exceptionally much. This also means that sometimes I sharpen them before they need to be, and sometimes I have a couple sessions of dull-ish blades, but not life-threateningly dull. I've been lucky enough to skate on well-maintained ice surfaces, so I'm usually not too concerned about bad ice dulling my blades.
I know my blades are getting dull when my stops (snow plow, T stops, etc.) become rather "slide-y". I don't normally use skidding on edges as an indicator, because skidding could be due to ice quality, or it can also be a sign of bad technique which would otherwise be masked by a sharp blade. If the skidding is actually due to dull blades, then those must be really really dull blades indeed. I try not to get to that stage.
I would avoid skating on freshly sharpened blades on a competition or test, it takes a bit of time to get used to the new sharpness and spins could be disastrous on freshly sharpened blades! But it usually takes me just about half an hour or less of deep edgework before I can get back to business. I've also found that if I go for too long between sharpenings, the larger difference between the dullness and sharpness takes longer to get used to. That's why I found a schedule of 24 hours of skating to be my sweet spot.
That said, I am an adult skater, competitive in spirit but recreational by definition, so I can afford to put up with dull blades sometimes. And it also has a lot to do with your own skating environment, personal preference, and psychological comfort. For skaters who are truly competitive, he/she may have different criteria for their keeping equipment in top shape.