I’ve had it all ways. The first job I got after graduation was offered to me on the spot. They flew me 600 miles, picked me up at the airport, and drove me to the facility. I talked to my potential manager, had a future colleague take me out to lunch, was introduced to the appropriate VP, the company president, and was offered the position before 5 PM.
With subsequent jobs, it varied, a lot. The worst delay was at least 4 weeks, during which they re-ran the newspaper ad, twice (yes, in the olden days, companies would run newspaper ads to fill employment positions, as strange as that seems). Not only was this the longest delay between my initial interview and being called back, it also followed the longest stretch of unemployment (or, as I like to view it, self-employment) of my career. After 4 weeks, they called me back in, and in another two weeks, I was offered the job. Apparently, they couldn’t find anybody better (and, obviously, not for a lack of looking). I held that job for nearly 15 years, when I left it to work for the world leader in the industry.
My experience pales compared to my wife’s. She interviewed for her current job (her ‘perfect job’) over the 4th of July weekend. They told her “we have two other applicants we have to speak with” and she came home. It was after Thanksgiving before they called back and offered her the position. Yes, it was a very nice offer.
It just goes to show, though, you can’t ever tell. In general, however, IME if it is a small company, and they like the way you interviewed and think you will do the job they have, they will offer you the job on the spot. The bigger the organization, the more hoops they hiring manager has to go through to hire anyone, so it just takes longer.