There was a 40 year waiting list for allotments in some parts of London 10 years ago[0], and 6 years ago the number of allotments had remained the same but the waiting list more than doubled from 800 to 1,765[1] so it could be longer than 40 years now.

[0] https://www.theguardian.com/money/2009/jun/02/allotments-sho...

[1] https://www.whatdotheyknow.com/request/allotments_and_waitin...