few points:
- will depend on feeding, heavily fed tank will require more frequent skimmer cleaning
- as someone mentioned before clean skimmer performs better
- average skimmer will extract waste easier from dirtier water, however not so good skimmer will stop skimming earlier then a better one
- go by the air water mixing provided by the OEM; the more L of air per hour the better. Obviously there is more to look at when buying the skimmer but this is one of the main parameters.
My skimmer needs cleaning every second day because i feed heavily, and skimmer is very efficient in waste removal.
more pics and description on page one.