Over what period must the no-confidence letters in the leader of the UK Conservative party be received?

A vote of no-confidence in Boris Johnson has recently (as of 06.06.2022) been triggered due to more than 54 letters (15% of all Conservative MPs) having sent letters to the chair of the 1922 committee.

One thing I have not seen in any coverage is over what period the letters must be received, if there is indeed such a thing!

Could I (as a Tory MP), for instance, write a letter day-one after a new leader is elected, and have it count for all of my time as an MP / the duration of their leadership?

Or is it required that the requisite letters are received in a timely manner? For instance the party-gate scandal has been ongoing since at least the end of last year, would a letter written in November 2021 still be counted towards the 54 letter total?

How about after an election, or if a vote of no-confidence fails (i.e. the incumbent is successful) and the leader is not removed. Does the letter no longer apply then, or will it continue? Should I write a new letter if a new leader of the 1922 committee is elected?