As more and more people deploy community network nodes a question that is being asked is "What information should I log as people use my access point". This is a tricky question to answer because there are several important angles to consider.
my personal recommendation (based on the practices of all of the isp's i have worked for) is to keep comprehensive logs for a finite amount of time for debugging purposes. after that, only maintain statistical records (mrtg type records which doesn't correlate data to people).
how long you think you should keep data is up to you. isp's normally do it for about a month, which is long enough that you should have all the debugging info you might need, but unlikely that a search warrent will be presented in that time. in our scenario i would think somewhere between a couple days and a week would be reasonable.
as a side note for the archives ... i/we am not advocating that we allow people to break the law, or even faciliate them breaking the law. we are working in the best interests of our "customers" (maybe members would be a better word?) to help make sure that their privacy and freedom are preserved.