Hi Bra
I applied mid January, and my invite to stage 1 was emailed the next day. I was then given the choice of dates for the following 3 weeks. Due to a holiday commitment I had to delay my initial test and didn't sit stage 1 until 6 weeks after the initial invite. I was sent a reminder after 4 weeks to ask if I was still interested in NATS. Probably best not leave it too long. If you do need to delay, best speak with HR, they're ever so helpful ;-)
BAW1T