Currently, I delete files 30 days/1 month after a client signs off on a job. I used to delete them after 2 months, but that was getting to be a bit much and taking too much space.
I have some sort of paranoia about deleting files any sooner than that, as often people will come back and want something additional on the original job (and yes they would pay for it), and it makes my life easier to have the original session to work with, instead of piecing things together.
This probably applies mostly to graphic design, video, and audio related things as they are done in programs that clients might not have access to, as word docs and text files are easily shared and re-edited/added to.
Anyways, just wondering what people do. Am I paranoid keeping things for a month? Or do you delete files as soon as a client signs off on it?