How to guess when a system was installed
 

I would like to know when a given linux system was installed.
May the right question is when the root filesystem was created, except it is a reiser3 fs.

any ideas ?

Probably the answer is in log files,

For Fedora Core 6, I have a install.log.syslog file in /root that tells me when first user was created, in /var/log I have a cron.3 file where (3) is the highest number in cron. files that tells me when cron first started, a few minutes after the user was added.
Search the logs, it is only logical that this is where such information would reside.

Actually,

I use imaging technology when putting together a system, after first install I create a compressed image of the operating system, then I start loading/compiling drivers, if things go wrong during such tasks I wipe (0's) across the partition and re-load the image in less than ten minutes and try again with different source or whatever. The files I mention above may not be so accurate in a case like mine. I do however have a Xorg.0.log in /var/log that suggests the actual build date of my system was two days prior to when the other two files were created, which stands to reason as I usually delete first user and create a new one whenever I load an image of a fresh build.