Like Mike Hallam, I have found that the vapour return line on my 912S returns 2-3 litres per hour to one of my Kitfox 5 wing-tanks (which can be a nuisance on long flights).
As to the original question, I do not know in practice because I am just about to fit a fuel pressure gauge! However, the Skydrive instructions state "At all speeds within the normal range (say from 2000 to 5000rpm} the pressure should remain in the range 0.15 to 0.5 bar. The normal pressure with a 912 engine is around 0.25 - 0.4 bar."
Hope this helps.