From what I gather "usually" the emails are sent out then 1 week later the soak test begins. Then the soak can last 1-2 weeks and it goes in waves of a few thousand people at a time. Now of course there were times where the email has gone out and then nothing happened for a few weeks due to a last minute issue that was found so a new build needed to be created. For the X though I doubt this will happen as there were builds released on T-Mobile, Sprint, AT&T and Rogers.
I could see the soak starting as early as Saturday night.