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.

[whatcaneyedo]

If everything else is optimal but your tank is still a slimy algae covered mess then it would be a good idea to upgrade the skimmer. But as Mitch suggests, adequate water flow throughout the display and the rest of the system is critical for getting waste to the skimmer in the first place. A friend of mine once had a very clean running 500gal and then downgraded to a 75gal that was always filthy. He used the same massive skimmer from the 500 but the water flow throughout the new system was so poor that very little of the waste ever made it to the skimmer to be removed.

[Myka]

I'd be upgrading the skimmer if I see nitrate and phosphate creeping up despite my best attempted to lower it. Some people use things like biopellets to help counteract, but even biopellets work best with a really good skimmer.