The GC is called everytime the gen-0/managed heap is full. But that will not necessarily 'release' memory or reduce a processes working set.

The finalizers on objects are called when

1) gen-0 is full.

2) System.GC.Collect() is called (Bad practice!!)

3) The Win32 CreateMemoryResourceNotification and QueryMemoryResourceNotification functions monitor system memory overall. If these report a low memory situation, the CLR may force a GC collect to free dead objects.

4) the clr is unloading an AppDomain

5) the CLR is shutting down (not same as process shutting down).

In v2.0, GC collections are no longer forced on AppDomain unload, but finalizers are still automatically run. (Many thanks to Chris Lyon for that one!)