About 6 months or so. I'd have to look it up for an exact figure (No, I don't remember my own T start date off the top of my head. My brain has a blind spot for dates).
I have noticed the same thing, it does go in waves. I think there were another 3 or so people that started in the same fortnight as me, and then nothing for a few weeks.
As one wave gets T, it seems a group of people that are ready take it as a cue to start making a move on the process. That's why I think we get the waves.
That and "if he can make a thread about starting T, so can I" - so even if the start dates are a little staggered, we still get the threads in waves (which makes it all the more noticeable).