For how long will the UK government / parliament elected on December 12, 2019 remain (serve?), if we're not counting any possible new non-regular elections?
Put in another way: When will the next regular general election for UK parliament be held?
The current law — the 2011 Fixed Term Parliaments Act — sets the next election as “the first Thursday in May of the fifth year after the last General Election”, which would make the next scheduled General Election the 2nd of May 2024.
The previous answer was correct at the time it was written.
However, the Fixed-term Parliaments Act 2011 was repealed by the Dissolution and Calling of Parliament Act 2022, which returned the (effective) power to call an election to the Prime Minister.
Quoting the linked Wikipedia page:
Section 4 of the Act provided: "If it has not been dissolved earlier, a Parliament dissolves at the beginning of the day that is the fifth anniversary of the day on which it first met." For the MPs elected in the 2019 United Kingdom general election, who first met on 17 December 2019, this means the fifth-anniversary date of 17 December 2024 and the latest possible polling day 25 working days later, which is 24 January 2025.
The 25 working days comes from the timetable laid out in Part 1 of Schedule 1 of the Representation of the People Act 1983, as amended.