Airlines recommend 3 hours. You can maybe cut this down by 30 mins if you can check in online (and print your own boarding card). For Heathrow I would always suggest that you aim to be there 3 hours in advance.

At London Heathrow (LHR) in 2026, the time required for check-in varies significantly by your destination and terminal. For international long-haul flights, you should allow at least 3 hours; for European flights, 2 hours is the standard; and for UK domestic flights, 90 minutes is sufficient. The physical check-in and bag drop process usually takes 20 to 45 minutes, depending on whether you use the self-service kiosks (common in Terminals 2 and 5) or require a full-service counter. However, in 2026, security wait times at LHR are the biggest variable, often ranging from 10 minutes on a slow day to over 45 minutes during peak morning and evening banks. If you are flying during summer holidays or the December peak, adding an extra 30 minutes to these recommendations is wise. Many airlines at Heathrow now offer "Twilight Bag Drop," allowing you to check your luggage the evening before a morning flight, which can reduce your day-of-travel stress to just a 15-minute security screening.