It generally depends upon the domain registrar, sometimes you can buy a expired after 15 days or sometimes it could take about 30 days.

I recommend reading this : How to Drop Catch an Expiring Domain Name

I used to buy and sell domains in college, and most of my good domains came from drop catches. If you really want a domain I recommend using Pool.com to backorder it as they have the server capacity to constantly pound the registrar on the drop date to get it before anyone else can.

I've never seen someone drop catching by hand beat pool