Announcing her departure at the Tory conference in October 2018 looks value at 12/1
I like this market from Paddy Power, a lot of the markets on Theresa May’s departure are when she formally stops being Prime Minister, this market is when she announces her intention to stand down (or loses a vote of confidence of the parliamentary party), not when she actually goes. For example if this market had existed for David Cameron, it would have paid out on June 24th 2016 when Dave announced his intention resign, not July 13th when he ceased to be Prime Minister.
I suspect 2018 will be dominated by Brexit and by the time of the 2018 Tory conference the shape of the Brexit deal, or lack therein, will be clear, despite the hopes and fears of many, the United Kingdom will be leaving the European Union in March 2019 and I think shortly after that Mrs May will formally go.
Much like David Cameron’s assurance prior to the EU referendum that he wouldn’t resign as PM if Leave won, we know Theresa May’s assurance that she will fight be Tory leader at the next general election will be similarly honoured, it just won’t happen because of political gravity will kick in.
In normal circumstances a Prime Minister announcing their departure date would only hasten their departure as their political authority weakens, but since 10pm on June 8th Mrs May’s authority has already been non existent, so she has nothing to lose on that front.
In front of the Tory faithful on October 3rd 2018 makes for an ideal opportunity for Mrs May to announce her departure date, Paddy Power imply there is a sub 8% chance that she will, my instincts thinks it is a bit higher than that.