Typically, the recommendation is every 30-45 hours depending on how often you skate. However, this can sometimes be flexed if your skates are sharp and you know how to identify a dull blade. There is the fingernail test. I’ll include a link to a hockey site (same test applies for all blades) Basically, if you run your fingernail (gently, don’t press it) up along the edges and your blades can shave the nail then they should still be sharp enough.
That being said, there are still some telltale signs that it’s time to sharpen your skates: you can’t ride an edge when you used to just fine, can’t even find an edge, if your attempting a 3 turn or other edge related move but instead of turning cleanly you’re sliding across the ice (sliding not slicing) then it is wayyyyy beyond time to get them sharpened.