Actually the complete search query was ‘heathrow +how+ much +time+ needed+to+go+ through+customs+and +retrieve+luggage +for+connecting+flight’. For that, the searchee ended up somewhere at January’s entries on my travels to Europe.
(Note this only applies to travellers who booked connecting flights on different carriers) My personal experience is that you need approximately 2 hours if you have don’t have any luggage on you: add an another extra hour if you need to wait for your luggage. Additionally, if you’re a European citizen (that is, if you have a passport from a country that is part of the EU), you may have a slight advantage going through customs since European citizens can go through an ‘express customs lane’. A couple of years ago, on my first return trip to Europe, my wife accidentally joined me in the lineup for European citizens (instead of going through the International visitors lane) and she was (kindly) reminded that she was in the wrong lane.
When you pass through Customs, make sure you verify the Terminal of your connecting flight. If you’re coming from an international flight (ie, a flight from North America), you probably will need to head for Terminal 1, which means you’ll have to take the train. Time is tight here: After Customs, go through Arrivals and directly turn left: I ended up taking the wrong turn and found myself in a mass of people who were waiting for loved ones. In any case look for the sign ‘Heathrow Express’.
If you’ve arrived at the Departure Terminal (1 or 2), the worst part is yet to come up: you have to check-in again and yes, it’s extremely crowded and busy. Worst yet, there’s security after check-in (30-45 minutes!) plus count on an extra additional luggage check at the departure gate. But if you’ve made it to the departure gate, you can (safely) catch breath.