NOTE: none of these actually delete the content on other people's pages, they just make your page less cluttered and delete YOUR content. so, it will delete your wall posts, but it will not delete the commentary of yours on someone else's page, only clear the activity line stating that you commented on person's page. read the descriptions.
this does it: http://userscripts.org/scripts/show/106969
it's just not automated clockwork, ie you have to each day/week/whatnot delete the stuff older than the selected post. i assume that is easily modifiable, especially if you run this script under a cronjob under linux.
this appears to clear/delete (as applicable) everything: http://userscripts.org/scripts/show/106713