Yes, the plugin will expire all posts older than 90 days if you set it for 90 days.
There are a couple of things to check here:
1. Have you waited a little while? The wp-cron job might not kick off immediately, since it runs at most once per hour. You might suddenly find everything has expired when you check back later in the day.
You can force an expiry by adding a line to one of your theme files (I always put it in the header, since that loads everywhere):
<?php do_TIEexpiry_all() ?>
Then just reload your main blog page (not the Dashboard) and it will run the expiry. Don’t forget to remove the line afterwards or it’ll run every time anyone loads the page, which is horrible for your server. Yes, I have done this… ??
2. The exact settings matter, obviously. Post status set to “Published” (box marked) or whatever’s appropriate for your requirement, radio button for “Categories to include in expiry checks” marked, category numbers list in the box there (comma-separated), “Expiry by age” has 90 in the days box and the “Use category filters” marked.
Screenshot of those settings here.
Let me know if it still doesn’t work!