This is one of those "how long is a piece of string questions".
What is good for a high-volume site may not be the same for a low-volume site with a lot of modules, and may not be the same for a medium-volume site with only a few modules enabled. Other sites running on the server may also be eating into the memory that APC has available.
My starting point is
I then adjust the settings for the server, but again, there is no one size fits all set of settings.
Find apc.php (probably located at /usr/share/pear/apc.php), and install it in the DOCROOT for your Drupal install. Lock it down to your IP address via Apache config. Edit the username/password inside, and then browse to it. You will then see your APC stats.
Reconfigure the shm_size parameter to minimize cache misses. Ideally, you want to give it just enough memory to never have a cache miss for a file, plus some additional memory for the APC user cache.
Optimize TTL to prevent cache churning. If you aren't editing files, you can bump up this number.
enable_cli is handy when you use drush a lot, so that drush uses the cached files from APC.
rfc1867 is handy for file uploads.
I also noticed the php-fpm in the title. This complicates matters, and is really beyond the scope of Drupal Answers. You may get better responses at Server Fault, as a lot of the system configuration settings don't pertain to Drupal directly.