They told me it could be up to six weeks before I heard about the next stage when I had my phone interview.. luckily I did't have to wait that long and received my email the Monday a week and a half after my interview
When you get the email you have to log on to book in a time, so they will need to have their time table sorted out I guess before they send any emails out... but I wouldn't worry too much - you might get your email today!