The filter should be turned 3/4 to 1 turn after the gasket touches the sealing surface. The nice thing about K&N filters is that they have a nut on the bottom of the case for tightening and loosening.
Before everyone gets all nuts, the reason you are not supposed to use a wrench (i.e. filter wrench) to tighten the filter is that you may damage/indent the outer case and give the oil a path past the filtration media. Using the K&N nut prevents that from happening.
