The site of Olympia Greece is big! Many organised tours leave a pathetic amount of time to get round it. If you’re just visiting out of curiosity then perhaps a couple of hours might be enough for you.
But if you’re an enthusiast about ancient Greece, then you’ll need a whole day. It isn’t that you can’t walk around the area in a few hours, but there is so much to take in. My own method was to walk around the site to get to know it, visit the museum to view the artifacts, take a break for lunch, and then walk around once more. That way on my second walk I had a much better appreciation of what I was looking at.
But even if your time is limited, don’t forget the museum. It’s one of the best you’ll see, including many models of Olympia Greece which show how it would have looked in its heyday. The pediment sculptures and metopes of the Temple of Zeus are masterpieces.
This map of ancient Olympia shows the main areas of interest.
Late Classical indicates features which were built up to the end of the 4C BC. Hellenistic is roughly between the 3C BC and 2C BC, before the Romans took over running the Mediterranean world.
Don’t forget that the structures in Olympia Greece have been built, demolished, destroyed, re-erected, renovated, extended, and the materials reused for newer buildings. It was constantly undergoing change and was never static.
Here’s a video showing a virtual 3D reconstruction of the main areas of the site.