Ideally (well, when I was taught) aircraft turning onto final at 12 miles keeps the final director nicely topped up, and sequencing off the stacks and calculating track mileage is done with this in mind (notice I say ideally)
However, as the downwinds for 09 are a "long" way from the Lambourne and Biggin holds (and a strong easterly wind can help
), it's easy to have a few more aircraft off the stacks and in the intermediate phase of the approach. You will then see aircraft turned onto the ILS 16 miles out......or further. Conversley, if there aren't many aircraft in the system or on final, they can be sequenced onto final closer to the airport (the directors can only vector to a minimum of 6 mile final)
The Heathrow Directors also have an airspace "manoeuvering" area that they must remain within at all times, hence if the final gets really long (say 20 miles), you can begin to see some very creative vectoring to keep stuff inside that area (there is also a controlled airspace step out there that, where the usual procedure of descending arrivals to 4000 feet may take them outside controlled airspace). You can run out of room down there very quickly.