How long are you allowed to sit in a Starbucks?

I frequent Starbucks when I know I need to buckle down and get homework done. I often wonder if there is a time limit. I always get a coffee or something so technically I am a customer. I sprawl out with my books and my computer and stay for 1–2 hours usually. Does Starbucks have a policy on the amount of time you stay? I imagine there are people out there who stay longer than me. Instead of searching for the policy or asking a barista, I figured I’d ask on here.