[kurt_]

> What to keep and for how long time ?

Try to make the data anonymous when possible (remove from log file all potential personnal data, not needed by your data process). If usernames are kept and can contain personnal data, if you don't need these data, remove them from log file.

Log only valuable data you need and only for the amout of time you need them.

> Is there any guideline here ?

In Europe, you have the obligation to retain all informations needed to identify the owner of an online publication.

"This Directive aims to harmonise Member States’ provisions concerning the obligations of the providers of publicly available electronic communications services or of public communications networks with respect to the retention of certain data which are generated or processed by them, in order to ensure that the data are available for the purpose of the investigation, detection and prosecution of serious crime, as defined by each Member State in its national law." - Directive 2006/24/EC - http://bit.ly/HxZcW

If you are based in Europe, you must refer to your national law for more informations.