From the 747 FCTM 4.21:
Maintain cruise altitude until the proper distance or time out for the planned descent and then hold the selected airspeed schedule during descent.
Since the flap speed for FLAPS 1 is 280 KIAS, and the descent profile has a speed reduction to 240 at 10,000', extending flaps at or near flap maneuvering speed should be assumed.
Further (4.22):
Plan the descent to arrive at traffic pattern altitude at flaps up maneuver speed approximately 12 miles from the runway when proceeding straight-in or about 8 miles from the runway when making an abeam approach.
...and on the next page:
Normal descents are made in the clean configuration to pattern or instrument approach altitude.
. . .
Extend the flaps when in the terminal area and conditions require a reduction in airspeed below flaps up maneuver speed. Normally select flaps 5 prior to the approach fix going outbound, or just before entering downwind on a visual approach.
Going forward to 5.7:
Flap Extension
During flap extension, selection of the flaps to the next flap position should be made when approaching, and before decelerating below, the maneuver speed for the existing flap position.
HOWEVER, the FCTM doesn't seem to address a constant descent approach. So, in order to be able to decelerate while descending, our FCOM used to say to start extending flaps at 17 miles instead of 12 (it is now silent on the subject...).