There has no need to be a general election until 2024. A lot can happen before then. A lot depends on how bad the economy becomes after the combined effects of Brexit & Covid, and whether or not it is starting to recover. If things get really bad by 2023, the tories will start of panic about the possibility of losing the next election, and some will start scheming about how to replace Boris.
And then the fun may start, because the tories are already showing signs of splitting into factions. We already have two clear parts --
(1) - The "slightly more caring" side, who want to mimimise Covid numbers / casualaties, and
(2) The "sod the public" side, who seem not to care about Covid casualty numbers, and think that "business profits" are the ONLY thing that matters.