Well, I agree with Fabulousity on the no sooner than 8 weeks, but that is just our personal opinions.
I do not go sooner than 10 weeks, and lately I go 10-12 weeks between touch ups, which seems to work for me at my current growth rate.
My hair is sort of fine and delicate to the touch, but verrrrrrry dense.
Deciding on a time frame mostly just has to do with your growth rate and how much overlapping will take place depending on how often you relax. For instance, if you relax every 8 weeks and your hair grows .5 inches / month, yet when you relax you get relaxer on at least the first 2 inches of your roots, you overlap and relax 1 inch of hair that has already been processed. So, all your strands will be doubly processed by relaxer. The goal is to limit any (unintentional) overlapping, which means extra processing, so your hair is easier to maintain in a healthy state.
This should make choosing how often is right for you a bit easier.
HTH.