Depends on how often you change lenses and how many shots you take in each outing and bursts.
I take about 3000 images per day of a race weekend across two or three bodies so they get a hammering and i clean as often as needed so normally about once a month or when it becomes a pain to edit hundreds of images in Lightroom 
It so easy to use the dust delete data especially at airshows when the slow shutters for prop blur give apertures that can show just how bad your sensor is in need of a clean that its worth doing and incorporating the use into your workflow to save loads of time,its not 100% as you do get a few new specs appear either from the mirror assembly helping move dust around or in the heat of summer throwing a little lubricant from the mirror box around as it becomes more fluid with the heat,or at least it seemed i had more trouble in summer at BSB than the colder months at the MX races.