It's in config-dist.php...
// Change the key pair lifetime for Moodle Networking
// The default is 28 days. You would only want to change this if the key
// was not getting regenerated for any reason. You would probably want
// make it much longer. Note that you'll need to delete and manually update
// any existing key.
// $CFG->mnetkeylifetime = 28;
If you are linking to Mahara there's no equivalent so that would need an 'easy' hack. You won't find the number 28 in the code in too many places
Whatever, make sure you extend the key at both ends or there isn't a lot of point.
Does this mean that key swapping isn't working still in some situation?