I'm surprised there is no official guidance - or at least no-one has posted a link to same on here
Go on then, I'll bite, Section 8 of Chapter 38 of the Queen's Regulations for the Royal Air Force:
http://www.raf.mod.uk/rafcms/mediafi...51C830E2B4.pdf
In short, Flt Lt and above. Acting rank if held for an aggregate period of at least two years during his service, or continuously for one year immediately before leaving the Active List.
Also, the rules for those who served during the Second World War are a lot more relaxed and acting rank up to 1954 counts.