Queen Elizabeth II, 93, is the longest-reigning British monarch in history, having been Queen since 6 February 1952. On 6 February 2017 she became the first British monarch to celebrate a Sapphire Jubilee, commemorating 65 years on the throne.
At the time of writing this question, Elizabeth II has been Queen for 67 years and 337 days. She is currently considered to be the sixth longest-reigning sovereign monarch with a verifiable reign of all time.
When will Queen Elizabeth II cease to be Queen of the United Kingdom?
The question will resolve as the date on which Elizabeth II dies, abdicates, is deposed, or on the date that the monarchy of the United Kingdom is dissolved, or if there is no longer a United Nations member state known as the United Kingdom.
Note that while Elizabeth II is also the reigning monarch in a number of other states and territories and various crown possessions, her position in relation to those bodies is immaterial to the resolution of this question.