_ac Cookie timeout settings?

After upgrade of one of my customer sites to to Plone they are experiencing cookie timeouts after ~30-45 minutes.

Question: Where is the setting for the timeouts? I should know this, but cant recall.

plone.session was not installed. I then installed plone.session to see if it helps to renew the cookie. It does not. I found a problem with plone.session in Plone 5.1, so see this as a warning unless it is fixed.