All you have to do is open your wp-config.php file. This file is usually located in the root folder, same directory where wp-includes is located.
define( 'EMPTY_TRASH_DAYS' , 1 ); //Integer is the amount of days
Change the 1 to 7 or however many days you want. If you don’t like the trash feature at all, then you can always set the number to 0, and get rid of it entirely.