If Boris Johnson does decide to throw the towel in - he would ride out the storm until the next election at least.
Given that he was really ill at the end of March and he has a young partner & a baby who he would want to devote more time to, I would be very surprised if he stands at another election.
My guess is that once the Covid smoke has finally cleared and the state of the economy has been fully established - there may be calls to bring an election forward, maybe 2023. Like Churchill - BJ might be able to navigate the country through some of it's darkest times, but like Churchill he might not be able to rebuild the devastation left afterwards.
As a prospective Tory leader - I would edge my bets at Rishi Sunak. Given the huge impact 'Black Lives Matters' has had on the UK, it would perhaps be seen as a token gesture to elect an Asian leader, but I do think maybe the UK perhaps does need a non-white candidate to lead the country. For me, there are too many people who continue to enjoy shouting 'racism' from the rooftops feeling that people from minorities are under valued and misrepresented - someone like Sunak would ruffle a few feathers and provide a calming influence.
Also, Sunak seems very plugged into the financial crisis the UK is under. As I've said - Sunak has a better grasp of the economy and the direction needed throughout the next decade. The financial impact caused by Covid is going to be much more of an issue once the virus itself has gone and this will be at the top of the agenda until at least the 2030s.
Pitri Patel wouldn't be a good PM - she is probably the best Home Secretary we've had since David Blunkett but I don't think her ideas and style would be the right fit as the Country's leader. Michael Gove would be my next choice after Sunak, but I have reservations that his style of leadership would be 'do as I say but not as I do'.
Kier Starmer does seem to be a promising opposition leader than his predecessor, but I do think Labour still has a lot of bridges to build since the mess they left the country in during the Blair/Brown years.
CJ