Gnome-keyring seems to ignore the settings for gpg passphrase caching duration. Steps to reproduce:

1) In dconf-editor edit the following settings:
- set '/desktop/gnome/crypto/cache/gpg-cache-method' to 'timeout' or 'idle';
- set '/desktop/gnome/crypto/cache/gpg-cache-ttl' to 5 (or some other value for easy testing)

2) Restart gnome-keyring-daemon (not sure if this is needed): 
$ gnome-keyring-daemon -r

2) invoke gpg2 or other program using gnome-keyring's gpg agent to get the private key, for instance:
$ echo "test" | gpg --clearsign

3) If the passphrase hasn't yet been cached, gnome-keyring asks for passphrase. If it has, it won't ask for the passphrase even if last time the key was used more than 'gpg-cache-ttl' seconds ago.

I would regard this as a security issue, because having these settings set, the user expects the pgp key to be protected with a passphrase in 5 minutes since last use (the default 'gpg-cache-ttl'), when in fact it isn't, and the user is unaware.