[AnchorAlumna]

DubaiSis, this year's complete schedule has not yet been released, but the 2012 schedule had:
Round 1: 20 minute parties
Round 2: 25 minute parties
Round 3: 40 minute parties
Round 4: 50 minute parties
Click on the Greek Chic link for more info.

Rounds 1, 2, and 3 have two days of recruitment.
Round 4 is just one day.
The reason for multiple days is, as HartofSEC mentioned, being able to accommodate the number of PNMs and sorority members in the houses. You can thank the campus fire chief for decreeing that a few years ago. While there are 4 new houses that will be open for 2013, most of the other sororities are still in their older, smaller houses. See the Alabama Panhellenic Preview Day thread for details on physical expansion of houses.