Cronjob Frequency and Emails Processed

Hi there

We are implementing the cron job as the current page visiting switch is causing multiple copies of emails to subscribers (we have a large visitor base). I just wondered the frequency in minutes (we usually process notification crons every minute or two to ensure rapid delivery), and how many emails the php script processes each time the cron calls the script? If it is the entire set, should we set the cron job to 5 minutes to let the script complete?

Many thanks