Right off the line it is a lot slower, but once rolling the 747-400 is gaining about 3 kts of speed per second.
Standard day, sea level, O mph headwind, MTOW of 875k, Vrot is around 185 kts. 185 kts/3 = 61.67 seconds. Typically there is some headwind so PAXboy is correct, the takeoff roll will usually be around 50 seconds.
