Santorini - How long to stay?

We were on the very popular Santorini for a day--coming off a cruise ship. It's a small but very beautiful island surrounded by other islands--most of which are uninhabited. We rented an ATV and saw the whole island while we were there.
It looks like a nice, restful place. But my wife and I don't do well for very long in quiet places as we're "moving" type people.

Sounds like me. I like to keep moving

I wouldn't go all the way there from Paris for less than 3 nights. However personally I'd get bored with anything more than 4 or 5 nights. However I know people that stayed there over a week and couldn't get enough of it. There are a couple of nice white washed towns, like Fira and Oia, which are nice to visit, plus some Greek ruins in ancient Thera. I then spent the rest of the time at the beach.
If you like boating and scuba diving, or to hang out at the beach for days at the time, or to explore the volcanic outdoors, then you might enjoy a longer stay.

I've been spending time on Santorini since 1985. There's plenty to see and do to keep you occupied for at least 4 days. I go for 4 weeks at a time and that's perfect for me, but I like to relax and take my time, enjoy the amazing views, make new friends, read a book or two, visit other parts of the island, hike, swim, eat and take naps. It really depends on what makes you happy.

So much depends on what you want for a Greek Island/Mainland Experience. Are you looking to party, nightlife, shopping, and a buzz or are you seeking tradition, culture and history?

Santorini may be the most "popular" and "travel agent Version of a Greek Island" experience out there. I've never been to Santorini and there is a reason: to touristy, to expensive, and to non-Greek.

If you like crowds, shopping, lots of tourists rather than lots of more locals then Santorini is for you. If you want culture, history, tradition, lower costs and a more Greek Experience you may to research another island.

3 days at least in my opinion. For such a small island, it has a lot to see actually. The caldera is absolutely stunning, especially in the towns Fira and Oia. This is also where the main tourist hordes congregate. The classical ruins of Ancient Thira are quite impressive and also located on a mountain top with great views and there were surprisingly few people when I visited this summer. The ruins of Akrotiri are unique because they are so old (about 1600 BC) and remarkably well preserved.
To some extend I agree with Tommyk5, it's also overrun with tourists, especially the annoying selfie stick waiving kind, who are only there to take the exact same picture from the guidebook to post on their instagram account. However it is also such a beautiful place, and you wont find anything like it anywhere in the world, that I am willing to put up with tourist hordes.

Santorini is simply spectacular. The caldera view is worth every penny. The expense however means I have to move along after 3 nights because that is all we can afford. Yes there are huge cruise ship crowds but they tend to congregate in Fira and Oia. If you stay in Firastephani you get that same caldera view, good sunsets and the bonus is the cruise crowds don't even know about it. Its about a 10 minute walk into Fira so its close enough that you can manage the cruise ship crowd interaction.
Just a kilometer from busy Oia you will find a traditional village of Finikia that is so quiet you might not meet more than 10 people a day.
Here is what Santorini is like.
Santorini http://www.flickr.com/photos/stanbr54/sets/72157632154800782/
One good idea is to pick a second island to go to after Santorini. Nearby you could visit Milos, Naxos or Paros. All are traditional Greek islands where there is a permanent population. While tourism is important to these islands most of the population is not tourism centric so you will get a true Greek experience. And all of it will be way less expensive than Santorini.
Naxos 2016 https://www.flickr.com/photos/stanbr54/albums/72157671589993576

Oh, it is a tourist magnet because it is fantastic! Other things to do.. boat. trip out to the volcano, if it is warm as swim in hot springs out there. Winery and tastings. They grow the vines very different than in France or USA. See the various beaches. The ancient ruins that are recently uncovered. Travel up to the peak and visit other smaller towns on the island. Enjoy the views, the food and the hospitality after the cruise ships leave. Loved this island and spent 3 nights.

c

What time of year are you planning to visit Santorini? If you're going to spend the time and money to travel there, I'd suggest at least three nights. That will give you enough time to explore the attractions on the island, have some R&R time and perhaps do some shopping. One suggestion though - don't ride the poor overworked donkeys up from the harbour.

We spent 3 nights and 4 days in a cave house, and wish we'd taken a day from Athens and added it to Oia.

We took a day and took a guided snorkel trip, which in retrospect wish we'd not spent a whole day at that. Diving might be another matter, but there's just not much to see on a snorkel trip. Although, seeing the island from the sea was worth it.

We spent another day hiking from Oia to Imerovigli, stopping at roadside stands for OJ and karidopeta me chocolad (sp?) along the way. Walking along the rough volcanic trails was the hardest 8km we've ever done, but it was amazing at every turn!

Another day was spent with a caldera trip, swimming to the hot springs, and a visit to Thirasia, all on a beautiful sailing vessel.

Every night we walked over to the Byzantine castle ruins for sunset, and afterwards explored in a different direction - art galleries, shops, groups of young and talented buskers, and restaurants everywhere, and no rhyme or reason to the street layouts. It was amazing, and I'd go back in a heartbeat. What did we miss? Plenty. Vineyards of Assyrtiko grapes and the wineries, the archeological digs and beaches of the south end, and lots of tiny churches went unseen. Sure, it's touristy, but for good reason. After 6pm or so, the cruise ships head out and leave the town nice and quiet. Our cave house was very private, with a stone terrace overlooking Ekklisia Agios Nikolaos. We enjoyed yogurt with honey and walnuts, pastries and OJ from a tiny market just up the marble Captain's Walk, which we picked up each day.
On our last night, we dined at Sunset Ammoudi Tavern, saw the sunset, and rode the donkeys back up the hill.

Like I said, I'd go back in a heartbeat. But, there's so much more world out there to see...

Spent 3 nights in Oia last April and that was just right. The weather was beautiful and there were no crowds. We did spend one day with a rental car touring the island although you can't see all the island in 1 day (or 3).

Would September/October still be good? I want to avoid crowds

We were there for four days one year, late September. Weather was perfect, four days the perfect length time. A lot to do? No, but enough for a few days.

"Would September/October still be good? I want to avoid crowds"

September should be good. Not sure about October as I've never been there at that time of year. One point to mention about crowds is that they will increase substantially on days when cruise ships are in port. I'm not sure if there's any way to avoid that, regardless of when you go.