This article describes how to change the session TTL for a specific port. In this example it is telnet.
config system session-ttl set default 1800 config port edit 1 set protocol 6 set timeout 3600 set start-port 23 set end-port 23 next end
The session timeout is in seconds.
Protocol 6 is TCP.
Protocol 17 is UDP.
If you do set protocol 0, the TTL is valid for all protocols.
Alternatively you can change the TTL per policy. Again, this is only possible on the command line.
config firewall policy edit <policy-id> set session-ttl 1800 next end
It is also possible to change the TTL per application using Application Control on the CLI.