The time limit for cron locks in Drupal 7 has been increased to 15 minutes, and is configurable from settings.php

#2319177: Cron lock time limit is too short and does not prevent multiple, concurrent cron runs has been backported to Drupal 7.

There was previously a hard-coded limit of 240 seconds for cron locks in D7, but this has been replaced with a new default of 15 minutes.

Sites that need to set a different limit can do so from settings.php as follows:

/**
 * Cron lock expiration timeout:
 *
 * Each time Drupal's cron is executed, it acquires a cron lock. Older releases
 * of Drupal set the default cron lock expiration timeout to 240 seconds. This
 * duration was considered short, because it often caused concurrent cron runs
 * especially on busy sites heavily utilizing cron.
 *
 * Use this variable to set a custom cron lock expiration timeout (float).
 */
# $conf['cron_lock_expiration_timeout'] = 900.0;