Good question. I applyed about 1 month before my IRT. However be very careful as i sent out some updated CV's recently and got phoned for an interview 1 week later. If that happened to you you will be 'stuffed'!
Bare in mind it's my 2nd interview in 3 years of applying the chance is fairly remote.
You would be better of waiting in case of delays etc.
good luck with your IR