I have no idea how long it lasted,
"21st Profile" lasted for 17 monthly(ish) issues( its cover price going up from ₤1:50 to ₤1:70 after 12 issues - Vol 1) It was published in from April '91, the issues are undated.
It was a sort of hybridised adaptation of the original Profile publications concept started back in the early 60's, but had the individual subject matter mixed and spread over a number of issues. The novel concept was that the layout was originally designed for the individual articles to be removed if desired and put into files without disrupting other subjects. Unfortunately,for whatever reason, this idea was abandoned in Volume 2.
The Editor was Eric Morgan and the Publisher Edward Shacklady ( Both of the Spitfire History published by Key) .There was mention implying, in the 17th issue, of Vol 2 going up to issue 24 and being available. Richard Ward did the Colour Profiles.
It suddenly disappeared, which was a shame. It was in my opinion a very good effort with no advertising within the main text and minimal editorial comment.