When does LabVIEW release a .dll file after Call Library Function call?

[rolfk]

Unless you call LoadLibrary() explicitedly the DLL will be released as soon as the LAST VI that contains a Call Library Node that references the DLL is unloaded from LabVIEW.

[dgholstein]

The fact LabVIEW keeps the DLL in memory is a huge programming feature!

When programming and debugging a DLL that creates handles to objects (libxml2 as an example), it's awesome to be able to keep those objects loaded in memory, make changes to your VI and rerun your VI - all without having to do all the things that got you to that specific state in your DLL.  Awesomeness squared, I tell you!!!

It means I can run and stop and change my VI without having to re-initialize my DLL memory space.  It's the advantage of working with LabVIEW as an interpreter.