We are experiencing a memory leak and upon close inspection I noticed that even after the hibernate session is closed the entities read from DB stay in memory even though there is no reference to them typically over one hour.

1. We are using only session level cache (default).
2. Our sessions are short lived (we are using spring OpenSessionInViewFilter)

I also ran the same query (in a different session) and now there are twice as many as before in memory which suggests that there is no caching taking place between sessions.

Is this normal? Is there a way to force hibernate to mark these entities for GC or at least shorten the time they are kept in memory?

I suspect Hibernate is using its inbuilt ehcache-failsafe.xml.Its guess but if it worls pls do rate.So you might create a new ehcache.xml and set the time to be 0.

Are you sure that there are no open references to the objects from within your application?

When the session is closed, the pojos get detached.
If they don't get garbage-collected, you should at first find out, who holds references to them.
Have you checked that?
(I would prefer yourkit java profiler to check for open references).