The desire for the governing liberals to inject new life into portfolios, re-position ministers for an aggressive fall sitting, and to take some potentially realized momentum from underneath the official opposition suggests that a potential shuffle would likely come before September 10th or soon thereafter.
Furthermore, it would not necessarily be advantageous to announce over the dog days of summer. Cabinet retreat would be a good time to have your new lieutenants embrace the coming legislative agenda of their newly appointed cabinet posts, and a retreat will take place likely the first full week of September.
With this in mind, one can discern that a movement of sorts would come in the window of end of August / early September. I would conclude that the week of August 29th – September 2nd is the most likely timing of a shuffle as this window best represents for the governing Liberals to achieve the desired boost from a shuffle.