Dust comes from everywhere - people, pets, open windows and mostly your heating/cooling system. If you have open rafters you need to clean them too when you dust. An unsealed concrete floor is a major dust maker (no clue why, but it is). Open windows (or doors) let in dust too.
You need to dust about once a week IMO. More often won't hurt of course. I keep my studio cleaner than my house, but then more people see my studio than my house
