much more than you can ever imagine. Or for that matter any written record !
The law also states that oral statements or commitments are also equally liable in some other contexts, but let's leave that at the moment.
Look at Comes et al. Vs Microsoft case where communication from as far as 1991 (or probably even earlier, I didn't look into too many exhibits) is coming back to haunt MS.
This doesn't imply that one has to be "extra-smart" while dealing with stuff bordering on what may be considered extreme positions in the distant future. Instead, just try and be good - "do no evil" !
Easier said than done ! :)