How long after takeoff for a Boeing 747-400 to reach cruise speed? Assume it is fully loaded (pasengers and cargo) and optimal conditions, almost no wind, etc etc...

How long after takeoff does the plane reaches cruise speed?

If he goes in a straight like on the axis of the airport strip (i.e. makes no turns), how far will have it traveled when it reaches cruise speed?
I did a search on FlightAware for 747-400 aircraft in flight, and sorted by the estimated time enroute. A Qantas flight from KLAX to YSSY came up. I'm assuming that for a 6500 Nm flight, at 13.5 hours, the aircraft is fairly heavy at takeoff.

The flight data shows a departure at 03:28:33, reaching an initial cruise level of FL300 at 03:52:59, after a continuous climb. It was at cruise speed at this point, which gives a time of 25 minutes after takeoff. Comparing the coordinates of these two reporting points gives a distance of about 180 Nm, which from the track is pretty straight.

If you just want to know when it reached a speed around that of cruise, this was about 10 minutes and 80 Nm earlier in the climb, when it reached a ground speed of about 480 knots.