Hiked this with my dad. Began at Prionia at 8 am, expected to be out quite late. Finished the hike at 4 or so if I recall correctly. Just be careful to go for Mytikas, NOT Skolio! At Skala you have to turn 120 degrees right, look for a 3 foot wide notch going downhill with arrows.
Got lucky with cloud cover to cool it off on the ridge. Easier than expected, shorter than expected. I almost wonder whether the local community puts this BS out there that it is extremely difficult to climb Olympus in a day in an effort to support the refuge...
Here is the GPS track (accidentally the last point is in Litochoro). 16 km before.
Very easy and pleasant , I did it more than 10 times this year.