We flew BA via Bangkok in April. Left Heathrow 11pm (scheduled 10pm), arr Bangkok about 3.15pm local (I think). 2hr 15 min on ground (should have been 3 hrs, but they wanted to try and make the time up). Arr Sydney 6.15am local (6am scheduled). I think the flight lengths were 11hrs 15 mins and 8 hrs. It was a bl00dy long time on a plane though!!
Yes I remember flying to Brisbane via Singapore with Singapore Airlines some years ago, stopping over for maybe 3 hours. I dont recommend it. It is a bloody killer. Spend an extra �50 and get yourself a decent nights kip at your stopover!