^^ What he said.. 2-3 litres, and to do it properly you do want to remove the return line, start the car and pour new fluid in while the old fluid is been pumped out of the return, once it comes out clean from the return she's all good. I only used 1.5L but it depends on how dirty the system is.

Easier if you have two people doing it, one person pouring in the fluid and the other turning on/off the vehicle!