when should I eliminate non-producing keywords in PPC

[charley]

For 12 of my sites I have about 300 keywords running in PPC. After a few months only a handful have produced positive ROI. Lots of people say that it is important to have many keywords but I am starting to wonder if this is really the case.

On the other hand, I guess I could try to keep tweaking the underperformers to see if I can get them into postive territory.