I can't find the reference but I suspect it will be in the UK AIP if it still exists.
This information was originally published many years ago because there were quite a lot of cases of crews being surprised by the need to hold. I was told a few times "we can only hold for 10 minutes and then we will have to divert." As you will know, holding routinely occurs at Heathrow and it would seem prudent to plan on being in the pattern for some time.
In the past delays of less than 20 minutes were classed as "no delay", however illogical that might seem. These days you will be informed by London ATC of the expected delay in terms of <10 minutes, 10-15, 15-20 or given an EAT.
Last edited by EastofKoksy; 24th Aug 2015 at 07:22.