How long until employers call back (after getting the job)?

How long (usually) does it take for an employer to call back? I had an interview and got the job on the spot. I was told to do a drug test down the street, which I took that day, and they would run a background check on me. This was a few days ago, and she said she would call to give me information about training, etc. On average, how long does it take to get called back? I know I passed the drug test, pretty positive I passed the background check (not sure if there's something there from when I was very young? But I don't think so). If it matters, I got hired by a major retail store. Thanks!