On May 27th, Boris Johnson appeared before the Commons Liaison Committee, a Select Committee made up of the chairs of the other Select Committees. The UK parliament website describes the committee's work:
It considers the overall work of select committees, promotes effective scrutiny of Government and chooses committee reports for debates. It questions the Prime Minister about policy, usually three times a year.
Johnson became Prime Minister on July 24th 2019, and so his appearance came 308 days after his appointment.
In addition, Johnson has already delayed his debut attendance to the committee three times, according to a letter dated October 23rd written by then committee chair, Dr Sarah Wollastan:
Frankly, I am astonished that, at such short notice, you are refusing to face detailed scrutiny from select committee chairs tomorrow morning. This is the third time that you have postponed or cancelled.
I have already conveyed the Committee’s deep disappointment that you did not appear on Wednesday 11th September, despite your suggestion of that date and your later reassurance that you would keep that commitment.
Our role as select committee chairs is to ask you detailed questions on behalf of the public and we planned to do so on Brexit, climate change, health and social care. It is unacceptable that you are refusing to be held to account.
This letter was in response to a letter by Johnson pulling out of his proposed October appearance, in which he suggested that his first appearance should come after the same timeframe in office as previous Prime Ministers.
Of the Prime Ministers who have been in office since Tony Blair's first appearance before the committee in 2002, how long was the period between their election and their first appearance before the committee? I can't seem to find full records of the committee's meetings.