Depends if you stop over, i did uk to bangkok, stayed there for about an hour, then bangkok to australia, think it was about 16 hours then another 8 but obv with stopover took a bit longer, well worth it though.
I did the same as becks but stoppped over in Singapore for 3 hours. Not too bad because they've got sun lounger type things to have a wee snooze on. Interstingly a parent let their child play the flute for those 3 hours so there's free internet if you can't sleep.