Hello ProfEdyB,
As I understand your question, you are trying to determine the minimum amount of time for which one can set the Elapsed Time Express VI and receive an accurate result. I do not have specific information on this use because it is, at least partially, determined by your computer's resources.
Generally, the Elapsed Time Express VI is not used for short (<1 sec) time periods. I suggest using a Wait (ms) or a Tick Count (ms) VI. Though these are also dependent on your system resources, they are more appropriate for timing short periods.
If you would like to achieve more accurate or shorter timing periods, you may want to look into switching from Windows to a real-time operating system. The main benefit of a real-time operating system is determinism. You can learn more about Real-Time by following the links below:
Real-Time Measurement and Control
http://www.ni.com/realtime/
Developer Zone: What is Real-Time?
http://zone.ni.com/devzone/cda/tut/p/id/3938
