Custom interval is not working

Note on input arguments:

You are using the filter callback function (FCB) with two input arguments, but the default is one.

In general you should replace:

add_filter( 'some_filter', 'some_filter_callback' );
function some_filter_callback( $arg1, $arg2 ) {

with something like:

add_filter( 'some_filter', 'some_filter_callback', 10, 2 );
function some_filter_callback( $arg1, $arg2 ) {

if you want two arguments.

You would also have to look at the apply_filters( 'some_filter', ... ) part to find the correct number of input arguments.

The problem:

When you check the source of wp_get_schedules() you find the following:

return array_merge( apply_filters( 'cron_schedules', array() ), $schedules );

so there’s no second $int argument passed on to the FCB.

The array that you return from your FCB is then merged with the default $schedules, where:

$schedules = array(
        'hourly'     => array( 'interval' => HOUR_IN_SECONDS,      'display' => __( 'Once Hourly' ) ),
        'twicedaily' => array( 'interval' => 12 * HOUR_IN_SECONDS, 'display' => __( 'Twice Daily' ) ),
        'daily'      => array( 'interval' => DAY_IN_SECONDS,       'display' => __( 'Once Daily' ) ),
    );

Hope this helps.