Ray,you will probably get a different answer from everyone.Some people empty it every few days while others will go a week or more.Your bio-load,feeding and how you run the skimmer-some people prefer a dry foam(less skimmate but much darker goop)and others wet(more skimmate but not as dark).I empty mine every 1.5-2 weeks(dry skimmate).Then again my collection cup holds nearly half a gallon.