For newbies like myself, these resources helped:
Finding the apc.ini file to make the changes recommended by c33s above, and setting recommended amounts: http://www.untwistedvortex.com/optimizing-tuning-apc-alternate-php-cache/
Understanding what apc.ttl is: http://www.php.net/manual/en/apc.configuration.php#ini.apc.ttl
Understanding what apc.shm_size is: http://www.php.net/manual/en/apc.configuration.php#ini.apc.shm-size