Shouldnt take longer than a couple of hours or so including filling back up and bleeding the system. if you drain it the night before, and give the expansion a good scrub with a kiddies bottle brush, then stick it in a bowl of boiling water with loads of bleach over night, making sure its submerged, give it a good scrub again and rinse in the morning, and it comes out very well! Obviously not like new as you cant get the brush to the edges, but you will be able to see what coloour your coolant is again!