@GayPareeee
, must look into what conditions make them use this stack as it seems to happen more in summer...
Things may have changed with regard to noise controls/curfews since my time but a quick discreption:
There are actually 4 inner holds for Heathrow - Lambourne (to the north east), Bovingdon (North West), Ockham (south West) and Biggin Hill (South East). Which one you used was dependent on where you were arriving from.
Roughly speaking Air Traffic Control would take aircraft off each hold in turn in batches of maybe three or four, but details varied.
You wouldn't expect anything in the holds overnight and the early arrivals that had night slots would normally ping straight in, no holding.
The holds normally started to fill up maybe from 0630 onwards as those aircraft that arrived with day slots and couldn't land before 0700 (I think it was 0700...might have been 0600) held until the clock ticked over.
After that throughout the day how many aircraft were in the holds varied, but it was rare to arrive at daylight hours and not go round a hold at least once, but if you got lucky and arrived in a gap in arrivals it could happen.
Usual cause of extended holding is either strong winds or fog...both of which reduce the landing rate (aircraft per hour).
HTH.