Duration: 4 days (Saturday afternoon to Wednesday afternoon)
Volcano tour - I know that a lot of people won't like tours, but I thought that the volcano tour was a very good way of covering a number of spots while at Santorini and I don't believe you can visit these places on your own.
Fira to Oia hike - Again, probably not for everyone, but I thought it was very enjoyable, although do note that there is no shade available during the entire hike. Plan accordingly.
Nothing disappointed me here. I did find Santorini very resort-like so it seemed a bit unnaturally friendly and clean depending on where you visit.
Lucky's souvlaki - not quite sure why we liked this place so much, but we did end up here a number of times. The souvlaki tastes very good and is very inexpensive.
Trani Taverna - very good food and if you luck out, a nice view to boot.
I think our duration was a bit long based on what we did. You may like to relax a bit more, but based on the things we did, I would suggest the following:
Day 1: Rent an ATV and drive the southern half of Santorini
Day 2: Do the volcano tour
Day 3: Do the fira to oia hike
That was sufficient for us in terms of things to see and do there. There are more things to do there, but that was enough based on our experiences.
Your duration is based on your interests. If you want to just relax, I would suggest a nice hotel with a view, otherwise, I think 3 days is a good amount of time to spend here.