bbPress.org

Here is a straightforward approach to abbreviating the time. Just put it in your functions.php file. You can modify the abbreviations as you wish:

add_filter( 'bbp_get_time_since', 'abbreviate_time' );
function abbreviate_time ( $output ) {
    $output = str_replace( 'minutes', 'min', $output );
    $output = str_replace( 'minute', 'min', $output );
    $output = str_replace( 'hours', 'hr', $output );
    $output = str_replace( 'hour', 'hr', $output );
    $output = str_replace( 'weeks', 'wk', $output );
    $output = str_replace( 'week', 'wk', $output );	
    $output = str_replace( 'months', 'mo', $output );
    $output = str_replace( 'month', 'mo', $output );    
    $output = str_replace( 'years', 'yr', $output );
    $output = str_replace( 'year', 'yr', $output );    
    return $output;
}