There is no absolute guideline, as it depends on the skater and the blade. My one kid likes blades very sharp, my other somewhat duller.
And, it's not just the edge: more often, it's nicks in the edges that are a problem. Kids who wear skate guards a great deal are more likely to have longer times between sharpenings.
Basically, if you run the edge of your thumbnail down the blade, it should peel a small strip off. Look for visible nicks as well.
However, if you're looking for timing: Ultima blades: 2 months, skating about 10 hours a week. Gold Seals: about 6 weeks, skating about the same amount, but, more frequently if freeskate testing or comps are upcoming, or if new jumps (for example, a triple whatever) or bigger and better versions of current jumps are being worked upon; the need for edge bite seems to be more necessary when the landings are shakier.
Fortunately, our sharpener is incredibly gifted at what he does, and is notorious for being able to minimize the need for sharpenings and get the most life out of blades ... which is why a visit to his shop can sometimes be startling in terms of who is hanging around getting their blades done (is that - OMG it IS!).
But, if you're kid is 10: you're going to outgrow those blades before they have no edge left for sharpening, anyways!