By default email import uses default HostBill crontab entry that is executed every five minutes. To make it execute more often (in one minute intervals), add separate crontab entry for email importer:
* * * * * /usr/bin/php /home/hostbill/public_html/admin/cron.php task=importPOP
Note: Make sure to adjust path to your cron.php to match location in your installation.