I passed basic selection at the beginning of June - I was then told I'd be going to Chicksands in the first half of July, or the third week of August. So it runs often enough, it would seem.
But yeh, just ask the chap who's been dealing with your admin at your local careers office - my local careers office knew well in advance the exact dates the selection course was on.
I think the speed with which you are sent to Int. Corps selection depends a little on if there has been any drama with your paperwork as well (admin cockups or they don't like your cousin Muhammed), so you may have a little wait on your hands between the selection board and Int. Corps selection.