I keep my negs for 1 year from the event, then I offer to sell the negatives to the customer and if they don't want them, they are destroyed, this is all spelled out in the paper work that I provide my customers when booking an event, my contract also states right up front that it is a model release as well and as such, I reserved the right to use images from the event in an appropriate manner. But again, even in the studio/lab I worked in, we kept them 1 year and followed the same protocol, it has worked very well for me and I have never had any problems.