How often should you use a clarifying shampoo?

I recently darkened my hair and I know that clarifying shampoos tend to fade the dye and dry your hair out. If I use product on my hair 3-5 days a week, how often should I be clarifying my hair? Also, is there a gentler way of doing this (so I don't strip my color)?How often should you use a clarifying shampoo?
I actually use a clarifying shampoo once every two weeks...but the most I would say is once a week... thats what I think .. I dont think its needed more then that unless you put alot of products in your hair during the week