On 17 December 2024, it will be five years since parliament convened after the last UK general election in 2019. If this parliament has not been formally dissolved by then, dissolution would at that point become automatic. A general election would follow after 25 working days. Strictly speaking, the last possible date for the next UK general election is therefore Tuesday 28 January 2025.
In practice, the next election is unlikely to take place then. For one thing, UK elections traditionally occur on Thursdays, though this is only a convention observed since 1935. (Five of the six elections immediately before 1935 took place on other days; so, in principle, could the next one.) Nevertheless, the fact remains that the next general election may well be as much as 14 months away.