With the proper flow, it should completely shed in a few days. With my low flow toadstool, it takes about 2-3 days after the polyps retract. For my med. flow toadstool, it takes about 1-2 days. As a note, my med. flow toadstool started out about 1/3 the size of the low flow, and now it's 50% larger than the low flow after ~6 months. So the sooner you can get it to shed, the sooner it's polyps will come back out and continue to grow in size.
Be careful what other corals are around the toadstool, because the shedding tissue is extremely toxic. If it happens to come in contact, it will most likely chemically burn the other coral.