Maybe 30 minutes for 7 rads in a 3 bed terraced. Ive done it a few times, found the easiest way is to
1. make sure heating is turned off.
2. stop the ballcock in the water tank for the rads by either tying it up with twine on to the roof supports (my preferred method as Ive left the twine permanently on it) or bridging it with a stick as per the link. You'll have to get up to attic for this.
3. Make sure all rads are open to completely drain system, lock any rads you dont want to drain.
4. attach a common garden hose with no attachments to the lowest point tap (theres a pic of one similar in that link). run the hose outside to a drain and then open the valve on the tap. Put a bowl/pot under the point where youve connected the hose to make sure you catch any leakage. If youre getting excess drainage stop the draining and try and tighten the hose on the tap.
5. When the water stops draining, carefully take the hose off the tap (may be some small leakage at this point). Then follow the instructions on the cleanser. lock the tap, release the ballcock in the attic and wait about 40mins-hour, the water tank will be full when the ballcock stops releasing water into the tank.
I cant remember whether you put cleanser in first or last but the instructions should be on the bottle. I think you then drain the system again after about 3 months to get rid of the sludge the cleanser broke down. Either way, the method for draining should be as above and even draining alone should get rid of some of the ****e. It does involve getting up in the attic so wear some old clothes and have a torch, and twine or bridging stick for the ballcock.
Bleed all the rads after refilling to release potential airblocks.
You may have an air blockage going out to the boiler too so make sure you bleed the drain on that as well if the heating wont come back on after refilling the system.
and so concludes my knowledge on draining a radiator system.