I always schedule them the same day if I can. Waiting between interviews does no one any good. I do a 45 minute with them for a one-on-one interview. I have a few the people they'd be working with ready to do more of a technical interview for an hour or so. If things don't go well after either, I let them know, and we part ways. If they do well, I put them in the yes pile. After one week from the first yes person, I choose the one I think best. Sometimes there is just one. When there's more, I choose the best fit and let the others know.
One time, when hiring a qa, I had two people who where both great. I really hated having to tell one of them no. A month later, a qa position opened on a different team and I passed on her resume. After she was hired, I sort of poached her from the hiring manager. She still reports to them, but she works entirely on my projects. But that's an aside.
My point is, a single interview is more than sufficient to hire anyone. The multiple interview crap, that's become so common, is done solely to stress someone out so they'll take a lower salary just to stop the stress.
Edit: Grammer
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety."