The length of the interview process varies. For a regular Joe Schmo, it can take anywhere from a few hours to a couple days or even weeks. It really depends.
@HRZone can give some great and more accurate feedback on this. In my case, I filled out an application online at 5AM because I was unemployed and awake due to a cold, and received a callback at 10:30 that very same morning.
From what I've heard, they give two interviews normally. In my case I came in for my first interview and was hired on the spot. I have to say ASANTS (All Stores Are Not The Same), but you can expect two interviews, maybe. As for a piss test, it may vary from state to state but I'm not sure if Spot does drug testing anymore. Shit's expensive.
The big thing you're going to want to think about is the questions they'll ask you. They will ask you things about "tell me about a time you did resolved a pressuring situation in a creative manner". It won't be that direct, but that's the gist of it. They're looking to see if you're a person who can be reasonably decisive and solve problems in a constructive yet somewhat creative manner. Spot likes people who can get shit done by thinking outside the box (as long as it's safe and sane) and with a smile on their face. It doesn't have to necessarily be work-oriented; if you're involved in social clubs, sports, volunteering, this is also applicable.
But yeah, just be cool, be yourself in the most professional way you can possibly be, make sure you're dressed appropriately (don't wear red and khaki to your interview, that's like going to a concert wearing a shirt of the band that you're seeing), and have a good attitude.