How to set entries to auto-expire after certain # of days from post date? Is there any way to set entries of an certain entry type to auto-expire after a certain number of days from the original post date?
There's no build in functionality for this, but you could make a custom plugin that automatically sets the expiration date expiryDate when you first save an entry with a specific entry type.

Another option would be to simply filter your entries from your template:

{% set entries = craft.entries.type('myEntryType').postDate('&gt;= ' ~ now|date_modify('-10 days')) %}


.

Edit:

Here's a description for the expression used as the value for the date_modify parameter.

Have a look at the syntax for parameter values in the Craft docs. And also how the date_modify filter works. now|date_modify('-10 days') returns a string which is then concatenated with the string '&gt;= ' and then passed as a value to the postDate() parameter.