I have recently transferred between Terminal 3 and Terminal 5 at Heathrow Airport. The ‘official’ time passengers should allow is 90 minutes, but the time it took me, from leaving the plane at Terminal 3 to getting through security at Terminal 5 was 45 minutes. I did the reverse trip a month later and it took only 35 minutes. The bus transfer takes 10 minutes and the only hold-up is likely to be at the traffic lights in the middle of the airport.