Hello,
since user objects in FIM Portal dont have a last modify/update date, like you already mentioned it is not that easy to extract this information.
One way could be to search the request history for the newest entry of a specific target.
So search all requests that updates a user and where the target is your user.
But: There are batch objects in portal (msidmCompositeType) that combines mutliple changes to multiple objetcs, you must handle them and search them seperatly.
I wrote a blog post on how to search within these batch objects:
https://justidm.wordpress.com/2013/10/06/fim-2010-r2-searching-for-request-details-in-msidmcompositetype/
In addition maybe FIM Reporting fits your need, but require additinal implementation.
/Peter