There is no definite answer to this question. Every skater is different and it all depends on what kind of use the skates get. When skaters start out shuffling across the ice, once or twice a season should be fine. When skaters start to use their edges more and dig into the ice, maybe 3 or 4 times a season. This is assuming skating once a week for one hour. When figure skaters reach a more advanced level, doing single and double jumps, skating 4-6 hours a week, once every month would be advisable. When Hockey players start skating one or two games a week, at a more advanced skating level, once every 4-6 games is a good bench mark.
Every skater is different and can handle different degrees of dullness. Of course, your 5 year old will not be able to tell you if their skates need sharpened. To see if blades are dull, you can simply feel them with your finger. Run your finger width-wise across the blade, not length-wise as you can cut yourself this way. You should be able to feel two distinct edges. Try feeling the blades before and after they are sharpened so you can get to know what dull and sharp really feel like.
Contrary to what many believe, getting blades as sharp as possible to make them last longer is not a good idea. Sometimes, the sharper the blade, the more difficult it is to skate and especially stop. Beginner skaters can struggle with scraping the ice or making snow and sharp skates do not help. The grooves of the blade just get stuck into the ice and the skater can’t push their foot to the side.