Anybody know what the retention period is for data pertaining to Y2K

Y2K seems like eons ago! There is no one answer but anything around preparation for Y2K could probably be safely disposed of. It's been 9+ years. I'd keep Y2K audit or exam materials for the same time period as other audit/exam materials. If you had litigation or negotiations with service providers over anything Y2K related, then you would especially want to follow the direction of your counsel for proper retention.

We kept our information, testing, etc for 2 exam cycles (roughly 3 years); then we destroyed everything.