When to upgrade skimmer?

[MitchM]

It really depends on your overall setup.
Skimmers only remove a small percentage of doc's, so if you have a substrate or coral overgrowth, detritus can build up, so a better solution may be to make sure that you have good water circulation and a clean substrate instead of getting a bigger skimmer.
Good water circulation will allow bacteria to digest DOC.
Thinning out coral growth will allow better water circulation.