Extending proftp's timeout

Hi list,

I want to extend the amount of inactive time a logged in user may be
allowed and still retain his connection.

It seems that if one is connected and a period of inactivity exists there
will be the necessity to reconnect.

I have looked at the proftpd.conf file and I do not see a reference
regarding connection time. Is there line I can add or one I can edit? Or 
perhaps this is function of my firewall...?

Thanks, Jon