Instructions said to add the flushing agent to coolant system and run engine with heater on high for 20mins (max 1hr) then drain after car has cooled down.
 
I went for a drive for the 20mins, but silly me forgot to put the heater on high, so I went for another 15mins drive with heather on high. I started this late in the arvo so by the time the car cools down it'll be dark and I'm working in my drive way, was thinking of draining the system tomorrow morning.
 
Would it be ok to leave the flushing agent in the system overnight?

We'll its dark now, won't be able to drain it until tomorrow morning.