When to arrive. We recommend getting to the airport three hours before your flight if you're travelling internationally, or two hours if you're travelling domestically or to Europe.
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We recommend that you allow the following minimum connection times: 1 hour for connections within the same Heathrow terminal. 1 hour 30 minutes for connections that require travel between terminals at Heathrow.
The General Guideline for How Early You Should Get to the Airport. Generally speaking, most airlines advise that you arrive at the airport at least 2 hours prior to a domestic flight, and at least 3 hours prior to an international flight.
Either way, three hours will be plenty of time. If your connection at LHR is all at the same terminal, you won't have to take the extra time to transfer between terminals. Even if you do have to switch terminals, 3 hours won't be a problem.
Allow between 30 minutes and 2 hours. Immigration queueing times have increased this week (some groups have been taking nearly two hours to reach arrivals from touchdown) but it's still very inconsistent so Peter's comment above is valid.
The peak times at London Heathrow Airport is from 10 AM to 1 PM, and 5 PM to 8 PM. Since London Heathrow Airport is the busiest airport in the UK, you'll find it jam-packed with passengers 24/7.
Generally speaking, most airlines advise that you arrive at the airport at least 2 hours prior to a domestic flight, and at least 3 hours prior to an international flight.
When you're traveling outside of the United States, we suggest arriving at least 3 hours prior to your departure. You must be checked in at least 1 hour before your scheduled departure. Additionally, we recommend being at the gate and ready to board 45 minutes before your scheduled departure time.
London's Heathrow Airport has launched a six-month trial—aptly called “Heathrow Timeslot”—where travelers can pre-book a security slot, meaning that you may be able to avoid facing any lines whatsoever at security and effortlessly zoom straight through to the lounge.
The Long Stay car parks at Heathrow offer 30 minutes parking for FREE, making them ideal if you're picking up or dropping off passengers. Blue Badge holders can park for free for 2 hours.
You cannot stay airside at Heathrow Airport overnight. You can however remain in the public areas and try and sleep (I wouldn't, as I would worry about my belongings getting stolen).