Pre-ordering will continue right up until release. From what I gather is that you want to know how long you have to pre-order in order to get into the Beta?

The Beta is a "paid" beta, which means that anyone who pre-orders will be able to participate in the beta test, giving feedback, etc. and then get the final game once it's released.  Since the main purpose of a beta is to get this feedback, we do it in stages, and if you miss one you'll have to wait for the next to take part.

That said, Beta 1 for Sins is currently set for early 2007, so you have at least a couple months to work with.