It took around 8 seconds from ingnition of three shuttle engines until liftoff. This time allowed systems check and after start checklist (just kidding) to be done until ingniton of solid boosters. Including first foot of ascent it took 9 seconds at most, which out of 500 total seconds of powered main engines flight represents less than 2% of fuel. Thrust/fuel flow was, excluding few seconds of reduction from 104% to 72% during initial ascent due to max dynamic pressure and last few seconds during which max g load was limited to 3g, constant.