From my tests, I am of the opinion that the value you set as the internal (Global Configuration> System> Session Lifetime) is used for both the internal and external interfaces.
You can check what value you get in the front. Use the following code:
echo "Session expires." JFactory :: getSession () β getExpire () / 60. min.
It seems to me that JFactory is responsible for setting the expiration time. It uses configuration time or, if absent, uses 900 seconds (15 minutes).
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