Flight due to arrive this morning at T4 at 9.25. Wheels down at 8.55 (early!). No queue at immigration (UK/EU passports) -- was through in 5 mins tops. This was a false time savings because I then had to wait ages for my luggage. Was on Piccadilly line by 9.45. So, even with short queues at immigration, 1 hour to get out of the airport still seems like a good rule of thumb.