Steve,
I'm no expert on skimmers by any means, but I will offer this.
Don Carner has written a 3 part protein skimmer discussion, easy to understand, and has some good overall information. There are some other good discussions as well on the topic at:
http://saltaquarium.about.com/pets/s...um/?once=true&
I'm not aware of any affordable test kits that would allow us to "measure" the amount of organic wastes in our systems, that would be a big help. I run my skimmer 12 hours at night, and timer shuts it off during the day. Some folks run them 24/7 and I suppose there are other personal opinions as well. All depends on whether you have a FO - FOWLR - or full blown REEF with few if any fish, and the number of waste producers in your tank, that will determine the amount of wastes produced. Also some people have the opinion that it is possible to "overskim" and others feel that is not an issue. Lots of things here ... type of mechancial and bio filtering, type of substrate, running carbon, phosphate sponges, nitrate removal via devices and water changes, protein skimming, chemical additives ... they all play a part on our water quality.
Two liters a week seems excessive, but you do have a much larger tank and larger fish than mine. My skimmer produces about a half cup per 12 hours of run time on a 55 gallon with sump, which may very well be undersized .. not sure. Excellent question though ... I'm interested in hearing other opinions on this topic !
Broomer