How soon is too soon to follow up on a job I applied for?

I applied in person on Saturday to a local music store. The people there know me VERY well, and I have been a good customer for well over a decade. I'm very well qualified, and they pretty much interviewed me on the spot. I was there for about an hour. It's Thursday, and the "help wanted" ad they had up is down, but I haven't heard back from them. Does it sound like that position has been filled? Should I call them or go in in person again? I just graduated from college and have been desperately searching for a job for months. I REALLY want this one.