More than 2 hours......especially at Heathrow.
Long queues for passport/ security checks, traffic jams, road accidents.....then parking and the shuttle bus........shall I go on. Chill out with a few coffees and be relaxed knowing you are there in good time. Especially long haul.
We were on the M5 recently, and got held up for almost 2 hours due to a vehicle fire on the opposite carriageway. Had we been flying from Bristol airport ( as we often do), we would CERTAINLY have missed our flight.
We always aim to be at the terminal 4 hours before flying. We have seen too many people miss flights by being late. Planes fly to schedule, and will not wait for you.