apply_filters ( 'jetpack_subscribe_comment_label', string $comment_sub_text )

Notes

You can use the filter to change the comment subscription text that appears below the native comment form when the Subscriptions feature is active:

function jeherve_sub_comment_text() {
        return 'custom text';
}
add_filter( 'jetpack_subscribe_comment_label', 'jeherve_sub_comment_text' );