In my 737 FLUF I start timing from the point of
rotation.
* 2 minutes for 747 and similar
* 3 minutes for 757 MD-11 and DC-10 (and not a second less!)
All born out from hard experience! <img src="eek.gif" border="0">
I put more emphasis on
time rather than distance. ATC uses ground distance rather than
air distance. Its the air distance between aircraft thats the factor here. What appears as a good 2 minutes ground-wise can be less than 40 secs in time given unfavorable airspeeds and wind.