When does it become Topless Sunbathing?

Okay people get your legal hats on. I am working on a double major in Lit and History so I have a reading list that would kill most people. I'm sunbathing on the beach and tanning my back with the straps untied. I order to highlight or read the book or write I have to get high enough on my elbows that everything is hanging out there, and seeing I don't have double D's the show on occasion. So far no one has said anything. So when does it officially become topless sunbathing? Does the nipple have to be exposed for 5 seconds, 10 seconds, 5 minutes?