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 site's design is much like the facade and interior of a store. Change things around too often and people find it hard to recognize it, find their way in it, grow fond of it etc.

Further, it take away resources that would be best used for the main purpose of the site.

A site should not be redesigned until at least a year.

If it's a personal website or blog, I don't think it's a big deal to change things up as often as you want. With businesses, I think gaining and keeping an identity is important. In that case, if you are going to redesign, give your customers plenty of warning ahead of time, too. Otherwise, they'll pop in and think they're on the wrong site half the time and then they're gone.

If it's a business website you should not change the design too often. It makes it confusing to visitors, especially those who find it difficult to browse the internet to begin with.

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.

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.

The look of the site-colour, header, font and layout become associated with the product. This branding helps the product to be recognised, and changing it so often is very bad because you are essentially redesigning the brand, the product. It would be like a big shop changing it's name every month, not good at all.

it's very confusing for users if you change the design, and google doesn't like a complete makeover of the site