Followed the directions to run diagnostics. Finally clicked "get started." Now all I have is a blank page with a very long url that starts with file:'/System/Installation/CDIS/MacOS%20utilities.app/contents/Resources anf on and on it goes. Is something happening and I just can't see it? How long does it take? Good thing I have my iMac to post here for now.
Apple Diagnostics take a long time as they are testing every memory cell, every CPU function, etc.
How long depends on how much memory, how much storage, how fast the processor is, etc.
Each version of Apple Diagnostics is a little different so I can't say for sure ir what you are seeing is normal or not.
In general, as long as you see the progress bar the tests are still running. When completed it will either say "No Issues Found" or a list of codes If you get a list of problem codes then your only option is a trip to the Genius Desk at the Apple Store for full up diagnostics and repair or replacement.
If you are unsure, try taking a picture of your screen with your iPhone and posting that for us to see what we can discern.
If we knew what it was we were doing, it wouldn't be called research, would it?
or a list of codes 
If you get a list of problem codes then your only option is a trip to the Genius Desk at the Apple Store for full up diagnostics and repair or replacement. 
try taking a picture of your screen with your iPhone and posting that for us to see what we can discern.
