Hi,
There isn't really a fixed timescale as it depends where the female is in the process.
And they do not always lock every time so it isn't a one shot deal. In fact it usually is a process that you go through for a couple of months.
This page explains it very well.
http://www.ballpython.ca/gallery/breeding.html
And remember to have your plans in place for incubating, housing and feeding the offspring early on in case the female moves the timescale up.
There is no greater feeling of panic then unexpected eggs when you have the incubator sitting in pieces in the corner of the room.
dr del