How quickly your knife dulls depends on many factors but, generally speaking, as soon as your knife seems like it is not cutting effectively, it is time for it to be sharpened. Most knives stay sharp for a few months. In some heavy use situations, like fish filleting or hours of use in a restaurant kitchen, knives may need to be resharpened after a few weeks of use, or even a few days.
If you are interested in learning how to extend the time between professional sharpenings, you can find some tips under Knife Care on how to maintain the edge of your knife.