The guided tour takes 45 minutes, and it is worth it to take the tour. You may have to wait for the next tour, though.
Walking up the hill from the carpark, walking around the headstones, etc. and the tour, I would suggest allowing no less than 2 hours. But, you could also spend a half day there, seeing all there is to see. There are some ruins down the hill that are supposed to be interesting, but we were not able to allocate the time. Perhaps others have, and they could better advise.
But, the Rock of Cashel is definitely worth the visit, and we are still grateful to Nancy on this forum for recommending it several years ago, when we were piecing together our Ireland itinerary. It is also quite striking to see it from afar, as you approach in the car.