When you log in to Myplace you start a login session, which is by default 2 hours long.
However every time you navigate to a different page in Myplace this session is extended to 2 hours.
This means that if you do not load another page for 2 hours, you will end up logged out.
On top of this when you log in to Myplace you also start a Shibboleth session. This is a shared login session that used by Myplace and can also be accessed by other University services (such as the library) and will allow you to access those services without having to enter your credentials again.
At the moment from the moment you login to Myplace via Shibboleth, a 4 hour shibboleth session starts.
During this 4 hour window if you go from Myplace to another service (that uses shibboleth for access) you will be automatically logged in, and the shibboleth session extended by 4 hours.
If you do not continue to access Myplace you will get logged out after 2 hours, but if 4 hours have not passed you will be automatically logged in to Myplace again (by Shibboleth).
However this will count as a "new" session so any thing you may have put in to Myplace and had not yet saved will have been lost.