It really depends on what kind of approach you are on, but if we assume you are on an instrument approach you will select the landing flaps setting when capturing the glideslope at about 8-10 miles out, then reduce the airspeed from 150 kts to landing speed Vref+5kts. The Vref is a speed affected by i.e. the landing weight of the aircraft and the flap setting you use. The Vref+5 speed will be kept all the way down until you will need to flare (lift the nose a bit) to make a nice and smooth touchdown.