There is no concrete rule for this. Perhaps you should more "quickly" review sessions 2-5 so that you refresh yourself on those topics. Spend a few days on each of those. Then you might start back to Session 6, and continue from there, spending about a week or as long as you need, on each session. The answer to overcoming social anxiety is persistence and repetition. The more consistent daily time you can spend on the therapy, the better off you will be.