It depends on where you live, every couple of months to half yearly. If you live ocean side then the salt spray can leave the windows in need of more frequent cleaning. Same if you live in a dusty environment.
The outside of your store including the windows are the first thing a customer will see when approaching your store. Dirty windows will be immediately obvious and portray an unprofessional image.
There are simply too many factors in play, depending on number of windows and height of the buildings the price will vary. Best to ring a few companies and get some idea of pricing for your situation.