I've read the manuals and scanned the posted messages, but I don't find an answer to my question:  How long should the takeoff roll be with 50% fuel load, full passenger and cargo load with GTOW of about 250,000 lbs?   Departing from KSEA on the 8500' runway, night time, ambient temp about 40 degrees, wind calm: I used second notch of flaps, set 2 of the 3 turbo switches to START, shut off all 4 bleeds, opened the L and R wing valves.  I held the brakes on while bringing up the power to around 65%, released brakes--and rolled forever while gaining speed.  I rotated at about 7500 feet but didn't unstick until after 8,000 and just barely cleared the approach lights.  I didn't get a good rate of climb for probably 2 or 3 miles from the departure end.  I know the 707 doesn't have nearly the performance of the 737, 757  etc., but should my take off roll have been that long?  Am I missing some steps in the proceedures?