Delete old private messages

[MyLibary]

I want like for example after 90 days private messages will be removed,
for example if someoen get private message at 10-10-2010 it will be removed at 10-01-2011 (90 days later +-)

Thanks.

[badcarb]

Any query or CRON option to prune messages older than x days?

I did an import and would really like to prune a bunch of old pm's but allow users to kep their newer messages