This is entirely dependant on what kind of knife you are using, the material your blade is composed of, and the number of times you use it on a daily basis. Your kitchen knife, with daily use, would need sharpening about once or twice a year. Send in your knife for sharpening the moment you feel difficulties handling the more difficult cuts. Your knife should be able to cut through smoothly, without requiring any additional force and, of course, without shredding the produce.