How long should it take to flush a 75 gallon water heater? It is just over five years old.?

Of course I should add that it never occurred to me that this needs to be done every year; this is the first time I've flushed it. I let the unit drain for about 15 minutes and didn't see a drastic difference in the color of the water coming out of the hose. The place that the reddish-brown water is most notable is in the master bath tub, which is one of the furthest outlets from the water heater. Any advice (besides calling a plumber)?