I have a friend who redesigns her site at least monthly. Granted the designs are fairly simple and rely on solid colors and a header which is controlled by CSS, but is it smart to make such drastic changes such as going from all green to all purple often?
The theme itself is open source so the cost is only the time she spends changing it. The header is something she makes herself using a basic graphic editing program. If it cost her she probably would keep them much longer. It is for a e-commerce site so I've been trying to encourage her to create a logo to keep static even if she changes everything else but she's not much good with that.Changing Themes would cost a bit, unless its a free or premium theme. Custom theme usually should stay for a long time and designs shouldn't change every month to month but yearly or so.