The rule of thumb for skate sharpening is to sharpen after every 30 to 50 hours of ice time. Use these hours as a rough guideline: need for skate sharpening can be greatly affected by use and care.
Continuous use of guards will keep skates sharper longer.
Walking across a concrete floor is fatal for your sharpening.
Failing to wipe blades dry and/or storing skates with plastic guards on can result in rust forming on the bottom of the blades, which will cause slippage.
A good test for sharpness is to scrape a thumb nail across (not along!) the blade. If the blade takes fine shavings, it is sharp; if the blade does not, maybe it's time to get the skates sharpened. Be sure to check both the inside and outside edges of the blade. Inside edges often get more wear than outside. For assessments and competitions, consider a sharpening a week or two before the big day. Not the day before!