If you haven't heard anything, please don't start stressing yet!
2 years ago we sat W 10+ on the Thursday, and got a letter inviting us for an interview towards the end of this week. Sat T 10+ on the Saturday, and got a call towards the middle of this week inviting us to interview.
Some boys at our primary school weren't contacted re: interviews until a few days after us; some had been contacted beforehand. I think it can take them a good couple of weeks to contact everyone to arrange interviews, whereas if you haven't done well enough to get an interview then you'll find out by the end of this week.
With the W interview, I had the impression that they had thought carefully about which teacher they wanted to interview DS1. Plus they had a boy who had gone to his primary school to take us to and from waiting rooms etc. So all quite carefully planned - which would take a while to sort out.
The T interview was on a Saturday, and there were loads of boys being interviewed - it felt a bit like we were on a conveyor belt! Apparently (historically at least - not sure whether Mr Kennedy has changed things) a Saturday T interview means that you are pretty much assured a (full fees) place - they just want to give you a quick once-over. Whereas a midweek interview means you could be up for a scholarship, or could be borderline for a place or not. Whether or not this is still true, I have no idea, but at least it gives us all something extra to speculate and stress about 
I'm afraid I don't have any numbers for 11+ at T or any at all for W (we only sat for T this time around) but if I find out from any school mum friends I'll let you all know.