The next CON poll lead in September looks a good bet
As the Wikipedia poll table shows there has been no recovery in the Tory polling position since Johnson decided to throw in the towel and step down as PM.
But could that change as we get beyond September 6th which is when the new leader will start. The previous experience is that new leaders can get a significant polling bounce.
Those of us who have been around long enough will remember what happened when Gordon Brown replaced Tony Blair in June 2007. LAB made a marked recovery in the polls that was only lost the following October when Brown blew it by not calling off plans for an early general election.
At current odds with Smarkets you can get about 4 to 1 on September being the next month in which there will be a conservative poll lead. I think that is a good bet.