FortiGate 500D - 5.4 Session-TTL Question

Hello,

Can someone tell me what the default session-ttl value is?  When I run config system session-ttl and show I get nothing.

(session-ttl) # show config system session-ttl end

I see it set in various firewall policies as "set session-ttl 0" which tells the policy to use the default.  If the default above is not set what happens?  Is there a built in default it's not showing me?  I do not want to set a default and break anything. 

The issue I am having is users are occasionally getting timeouts when using a web application from outside > DMZ.  Sometimes it works fine and other times they get timeout errors.  When looking at FortiAnalyzer traffic logs I see some sessions as "firewall action: close" and others as "firewall action: timeout" 

The current policy that I am having issues with does NOT have any session-ttl values set.  The service (http) under config firewall service custom has "set session-ttl 0" which again points back to a default that isn't set.