Persistent Shopping Cart – What Does It Do in Magento?

The time duration for which Item saved into cart depends on two parameters

1. Magento Admin: System -> Configuration -> Web -> Session Cookie Management Cookie Lifetime
2. PHP.ini - session.gc_maxlifetime

Here your Session Cookie Management Cookie Lifetime should be less than the session.gc_maxlifetime.

The session.gc_maxlifetime will be honored first and then cookie lifetime.

So once the session is expired the cart will be emptied.

That's how I thought it should work as well (see also: What does "Persistent Shopping Cart" do?)

But after inspecting the code, you are right, the persistent_shopping_cart cookie is only set on login and logout, and used for the "remember me" feature to automatically log in customers.

So the feature does not quite live up to its promises and I am not sure if this is intentional or a bug.