I'd say a year, because of the different seasons!! This is a good idea (from another pinner): Every January, I turn all of the hangers in my closet backwards, like this. It takes about 60 seconds: Each time I wear an item, I hang it back up in the normal fashion, which takes zero effort to remember or do. In June, any item that is still hanging backwards has not been worn in 6 months, and I usually donate it to Goodwill unless I can come up with a good reason to save it.
Here's a great blog on cleaning and maintaining your home. I love how she has a notebook and goes through the house like a prospective buyer. List everything that needs attention. Why should your house only look great when it's up for sale. By breaking it down into manageable lists, everyone can have a beautifully clean, well maintained home!