At the moment, that is not so easy to change, and this cannot be done via plugins which means you will need to modify this with every version upgrade. We will be rebuilding the paid listing features in the future (though it is hard to say when) to be more flexible, I would not recommend making this change given the current circumstances.
if you don't have solid understanding of PHP, MYSQL, JAVASCRIPT do not attempt this, these changes are NOT supported or recommended.
You will need to edit the MySQL database structure via phpmyadmin to change weeks to months. (huge downsides here as you will not be able to count on future updates to work properly)
files that will require hardcoding edits:
/sources/user_cp/user_premium.php (many locations)
/sources/misc/new_day (several locations)
skins/yourskin/premium_form.html (many locations)