Every time I get around to looking at it, which at the moment (as we have plenty of numbers) is about once a year
Technically it's every three months (that's our policy, I've just been lazy about implementing it), but when it does happen we send out at least 2 emails over a 2 week period before finally deleting, giving you the option to dial it and reset the timer.
We never delete your principle SIP number if you have one (that's the 0844 986 or 0844 933 which is on your MySIP Account page). We treat those as subscriber numbers, and they'll last forever. They can only ever be used to terminate to your VoIP User SIP device.
Dean