//I also didn't think Charles could be officially called King until his Coronation - but I was totally wrong!//
Indeed. He became the King the moment the Queen died.
Fifteen months seems to be about the usual period from accession to coronation:
Edward VII: accession 22 January 1901; coronation 9 August 1902—18 months and 18 days;
George V: accession 6 May 1910; coronation 22 June 1911—13 months and 16 days;
Edward VIII/George VI: Accession (Edward) 20 January 1936; coronation (George) 12 May 1937—15 months and 24 days;
Elizabeth II: accession 6 February 1952; coronation 2 June 1953—15 months and 25 days.
It could be argued that the Queen’s father (George VI) waited only six months to be crowned. However, his elder brother (Edward VIII) acceded to the throne on 20th January 1936, but he abdicated on 10th December that year without being crowned. The coronation of George VI held the following May was in fact the ceremony that had been in planning for Edward since his accession fifteen months earlier.
It could well be that the coronation of Charles is held a little less than 15 months after the Queen's death. Fifteen months takes us to Christmas 2023 or thereabouts. I imagine the organisers will hope for good weather and there's a greater chance of that in summer than in December.
Plus, of course, a lot of the pubs will be booked up with Christmas do's, so they may have difficulty finding a venue for the post Coronation booze and finger buffet. :-)