Hair coloring and perms definitely takes a lot out of your hair, so I guess I would say that every two weeks would be way too often. I guess dyes without ammonia are safer but I still think every two weeks seems too often. And I don't think the colors work as well anyway, at least from what I heard. There are dyes called hennas that are made from vegetables, but you can normally only get darker colors (reds and purples), so you couldn't do blonde.

One way to getting more regular "makeovers" is to get a haircut that lets you style it in different ways (not easy, but a good stylist can do it). You can also use things like barettes and ribbons to liven things up.