I’m a Girlwatcher

Really really really quick question. Help me out.
How long must I wait before turning around to check out the rear-view of a really good-lookin’ dark-haired, fair-skinned chick/dame/broad/skirt/babe who passes by wearing a little black dress and cowboy boots?
Must I count to ten before turning around? Must I wait until half a block separates us?
What’s the protocol?