Strap in lads! We're in the for the long haul!
This is probably the length of time he figured he needs to completely strip the country of its assets and values, and hand over the profits to his buddies.
Or he wants to add another hyperbolic achievement to his reputation.
Johnson has as much chance of beating Mrs Thatcher's post-war record as I have of going to the moon.
Mrs Thatcher was clever. She had a team of very good party managers (whips) who kept people in line and made sure anyone who was unreliable was dropped (gently, often with a peerage), she was also very good at remembering MP's wives, children etc. names, and could be very sympathetic when an MP had personal troubles (wife divorcing them etc.). Obviously, like all long-serving PMs, they become complacent and she went full mad with the poll tax.
Johnson treats everyone with contempt. He has no real close friends, only people he uses and discards (like wives). He has no real allies like Mrs Thatcher did, and his authority will drip away, hopefully before Xmas.