@smfefe70,
You can use the below Code For it:
add_filter( 'wpforo_usergroup_get_flood_interval', function(){ return 15; /* seconds */ } );
Solution by inserting the code in Themes functions.php file
Add the below code at the end(bottom) of the current active theme functions.php file
Option one: open functions.php file from wp-content > themes > current active theme.
Option Two: Go to Dashboard > Appearance Theme File Editor, select Theme Functions(functions.php) file from Theme Files Bar.
Please mention that the code can be lost after updating or changing the theme.
Solution by using Code Snippets Plugin
Install and Activate Code Snippets plugin( https://wordpress.org/plugins/code-snippets/).
Go to Dashboard > Snippets > Add New.
Insert the code in code field:
Check that the new created snippet is active from Dashboard > Snippets > All Snippets.