You may of considered reading the PureFTPd documentation at http://download.pureftpd.org/pub/pure-ftpd/doc/README
- '-I <timeout>': Change the maximum idle time. The timeout is in minutes and defaults to 15 minutes. Modern FTP clients are trying to fool timeouts
by sending fake commands at regular interval. We disconnect these clients
when they are idle for twice (because they are active anyway) the normal
"If at first you don't succeed; call it version 1.0"