I'm a 2L and I heard within a week to 10 days for callbacks (generally they notified me by telephone). I'm sure it is fine to contact someone at the firm; usually they mentioned to me at the interview when the next hiring committee meeting was so I would know when to be expecting to hear from them. I wasn't dinged after any pre-screens, so I don't know whether not hearing after 2 weeks is a bad thing or not. After the callbacks so far it has taken less than a week to get offers, but I've heard that process can be staggered as firms try to manage their yield.