I think there is no recommended amount of memory, the more users and photos, the more memory the server needs to process requests.. If you have little memory, you should receive an email that looks something like this:

PHP Fatal error: Allowed memory size of 1073741824 bytes exhausted (tried to allocate 668995584 bytes) in /home/domains/bbbbbbbb.com/web/public/inc/utils.inc.php on line 1138