I had three callbacks before Sept 1 (all NYC BigLaw) - I heard from 2 of them at the same time (w/in a day of each other) a little over two weeks later (phone offers). The 3rd (I'm not saying which...let's just say it's the biggest law firm in the world) has STILL not gotten back to me. I contacted the recruiting director last week, she apologized and said the hiring committee hadn't met yet. Uh, yeah, it's been a month. Anyway, I had 4 more callbacks this past week - Monday through Wednesday - and I got two offers on Thursday - one firm took three days, the other took two. One of those firms meets twice a week - Tues & Thurs - to make hiring decisions, so candidates hear really quickly. The firm at which I used to paralegal, for another example, meets once a week. It's possible that at smaller firms or offices, they meet even less frequently. Another factor is also that when a firm makes a decision to hire you, they'll tell your screening interviewer/person you know or connect with at the firm to call you. If that person gets busy, they may put off calling you for a day or two. This would obviously delay you hearing.
Basically, I don't personally believe there's a blanket rule. If you interviewed on Monday, I wouldn't sweat it if you haven't heard - it's possible that their hiring committee meets on Fridays and therefore you wouldn't get a call til next Monday. Alternatively - it IS possible that if you're right around their cutoff (in terms of numbers, etc. - aka a "fringe person") then a firm could make you wait longer; they may wait to interview other candidates from your school, etc, to see how it shakes out. No matter what I would call or email the head of recruiting if you haven't heard in two weeks.
Wow this is really long. Just remember to breathe! It's so stressful, but it will be over SO soon.
Good luck! Let us know how it goes...