It is never too early or too late to prepare your tenure and promotion or post-tenure material. I have read hundreds of portfolios as a department chair, dean, and provost. I have seen outstanding ones and mediocre ones. However, what strikes me most are the ones I have read from well-qualified applicants who fail to represent themselves well. Moreover, I have found that women tend to underrepresent their achievements and hold back on communicating what they have accomplished. In these cases, I think "why didn't they ask for advice?".
There is a way that administrators read portfolios, and you are best served by building your portfolio with an eye to how they read them. Candidates generally need to learn how to organize, write and prepare their material in a manner that provides the administrator with the smoothest read. Conversely, a difficult-to-follow portfolio is a bane for an administrator.
I have also found that it is never too early to start. If you are at the start of your tenure track, it is best to start building your portfolio immediately and work on it throughout your pre-tenure years. Having someone with you from start to finish to help keep you on track and adequately focused is the key to success.
If you are coming up soon, there is still time to strengthen your portfolio and get feedback on its contents and organization. Likewise, the same goes for post-tenure review and promotion.
The summer is a great time to work on your portfolio. I would love to help. I will see you from start to finish. I will be with you every step of the journey.