How to get subscription key or id using Woocommerce Subscriptions [closed]

I figured out the answer, so I thought I’d post it. My code looks like this now and it works:

add_action( 'processed_subscription_payment', 'updatedays', 10, 2 );

function updatedays($user_id, $subscription_key)
{  
    //do what I need to the sub key
}

There really needs to be more examples on the Woothemes documentation though..