At nearly every SES conference, I hear it mentioned that analytics must track 60-90 days out in order to maintain a close watch on latent visits and conversions. In a meeting I had today with a large non-profit based website, the issue of latent tracking over the longer term came up... My question is - is it valuable to track via IP address over the long haul (100-300 days) or are the changing IPs of many ISPs making that data largely irrelevant?

Does anyone here track over the very long term and what results have you seen? Are there significant percentages of visitors that do return for a conversion or action of any kind after 3-4 months?