Up to GnuPG 2
The user configuration (in
~/.gnupg/gpg-agent.conf) can only define the default and maximum caching duration; it can't be disabled.
default-cache-ttl option sets the timeout (in seconds) after the last GnuPG activity (so it resets if you use it), the
maximum-cache-ttl option set the timespan (in seconds) it caches after entering your password. The default value is 7200 (2 hours) for both.
Set it to a year or so – say, 34560000 seconds (400 days) – and you should be fine:
But for this change to take effect, you need to end the session by restarting gpg-agent.
If you want to limit to your session length, you'd need to kill the daemon at logout. This is very different between operating systems, so I'm referring to another question/answer containing hints for different systems.
You could also restart the
gpg-agent during login, but this does not limit caching time to the session length, but logins of a user. Decide yourself if this is a problem in your case.
GnuPG 2.1 and above
In GnuPG 2.1 and above, the
maximum-cache-ttl option was renamed to
max-cache-ttl without further changes.