My setup took just under 1 litre (as in only just under 1 litre!)
That's 2x graphics cards, 1x cpu block, a 360mm radiator and a 250mm EK tube reservoir (which is about 80% full)
For most loops, 1 litre is probably enough, but it's always worth having some spare, in case of leaks or if you decide to do something different later on.