Just to prove everyone's experience is different :-)
When I first filled my system, I found it took about 4.5l before blowing out to the catch tank.
I now find it will take more like 5.5l, with minimal blow out, even on a track day. My engine is a standard 1.8K (currently) with the Caterham DS system. The catch tank is mounted up front in the upper cruciform. A couple of guys at Caterham advised me to fit it there, to minimise the blow out.
So, 4-5.5 litres then :-)