Why is background check takes so long. I already have verbal offer?

So I applied this internship at xyz firm. I got a verbal offer the next day. It was quick. They then asked me to do finger prints the following Monday. It's Thursday night I still haven't heard a result back. I called them on weds they said everything is fine nothing is wrong they are just waiting. But I heard from other employer the HR that for him it only took a day or so. My question is they said not to worry but I'm still worried. Is it typical? Should I call them again? Help plz