Use a filter hook to change the length of the event excerpt displayed in the event listings.

event_listing_excerpt.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter( 'FHEE__content_espresso_events_details_template__the_content', 'my_custom_ee_excerpt_length' );
function my_custom_ee_excerpt_length( $excerpt ) {
    $excerpt = espresso_event_content_or_excerpt( 20, null, false );
    return $excerpt;
}

event_listing_excerpt_also_change_more_text.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter( 'FHEE__content_espresso_events_details_template__the_content', 'my_custom_ee_excerpt_length_and_text' );
function my_custom_ee_excerpt_length_and_text( $excerpt ) {
    $more = ' <a href="' . get_permalink() . '"';
    $more .= ' class="more-link"';
    $more .= \EED_Events_Archive::link_target();
    $more .= '>' . 'my more text' . '</a>'; // change text on this line of code
    $excerpt = espresso_event_content_or_excerpt( 20, $more, false );
    return $excerpt;
}

instructions.md

Usage notes

You can add the code to a functions plugin or into your WordPress theme's functions.php file.

You can change 20 to a different value to suit your needs (number of words).