Hi Kira, there are several routes to Old Perithia from Pantokrator; the shortest is perhaps the most difficult, scrabbling down steep hillsides and ravines where in places the path can be barely visible. Perhaps the best route is one that sticks mostly to the main "fire" roads - the unsurfaced rough tracks that criss cross the mountain. You will still get breathtaking views and beautiful scenery.
By the end of September, with the somewhat cooler weather, this treck is not arduous, but still take plenty of water. I think a couple of hours would be sufficient for the fire road route for an average walker. Don't forget to take protective clothing against the possibilty of intense sun or rain!
On the reverse side of my road map is a large scale detailed map of the North East Pantokrator region showing all of the hundreds of kilometers of roads, tracks and footpaths - just the area you are interested in.