Hike from Fira to Oia

I wouldn't. But your tolerance for crowds and your interests may vary from mine.

You're unlikely to start walking at 7:00. Santorini is a tender port, meaning it will be a while after you get there before you finally get a tender to take you and everyone else onto the island. The tenders drop you off hundreds of feet below Fira, so you have to add the time to walk up, donkey up (please don't) or line up for the cable car. Once on the trail, as there is a cruise ship in town, the trail will be crowded. You'll be hard pressed to make 2 mph. Buses and taxis between the two spots are not entirely reliable, and will (of course) be crowded.

As you can tell, I don't enjoy the crowds of Santorini that much. If I only had those few hours, I would take the cruise's excursion to Oia (first onto the tenders, guaranteed bus). Once in Oia, walk down to Ammoudi (the little fishing village below Oia). Walk through the village to the beach at the far end. Its not a sandy beach but a jumble of rocks that you can sunbathe on, and swim out to an island with an old Orthodox monastery. The best place I've ever swum in the Mediterranean. Then back to Ammoudi and eat at any of the delightful fish restaurants on the harbourfront. With whatever time you have left, you can go back up to Oia and poke around. Get the excursion bus back to the ship, if possible, or take a cab/bus as in your original plan.

Here's a Google Map of where I'm describing: https://www.google.ca/maps/place/%CE%A4%CE%B5%CE%BB%CE%B5%CF%86%CE%B5%CF%81%CE%AF%CE%BA+%CE%A3%CE%B1%CE%BD%CF%84%CE%BF%CF%81%CE%AF%CE%BD%CE%B7%CF%82+-+%CE%86%CE%BD%CF%89+%CF%83%CF%84%CE%B1%CE%B8%CE%BC%CF%8C%CF%82/@36.4588792,25.3692159,635m/data=!3m1!1e3!4m5!3m4!1s0x1499cdd20894a23f:0x7f011c8a631881a3!8m2!3d36.4204261!4d25.4302839?hl=en

And here's a Google Street View picture: https://www.google.ca/maps/@36.457922,25.3721569,3a,75y,159.73h,88.31t/data=!3m8!1e1!3m6!1sAF1QipPjeqpPtcf7H8Cbv0_mIZdpFfPGd4HvX-pzjymc!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipPjeqpPtcf7H8Cbv0_mIZdpFfPGd4HvX-pzjymc%3Dw203-h100-k-no-pi0-ya346.19308-ro-0-fo100!7i10752!8i3630?hl=en

I'm just commenting as a reminder to follow this post as I've wondered about it as well and want to do it next time I'm there.

We only had a day in port on our cruise in 2017 and decided not to do it as we had plenty of other things to do. But one thing I remember well was that it was a zoo trying to get transportation back to Fira in later afternoon as it seemed that all the passengers on all 3 cruise ships in port were trying to get back to Fira the the same time. It may be smarter to take a bus or cab to Oia and walk back. However, while the elevation between the two is a bit of a roller coaster, I've read that Fira to Oia is more downhill.

If we are hiking on September 29 would not the crowds be significantly less and our ability to get a cable car up the hill and a bus/cab on the way back to Fira a bit easier?

Unless hiking is a major priority for you, I think your time would be better spent if you forego the hike and concentrate on visiting one or two chosen locations on the island, such as Oia, Akrotiri, or perhaps one of the two black sand beach villages, Perissa and Kamari.
In pre-covid years there has been a water taxi to take people from the tender dock below Fira to Amoudi Bay below Oia. Hopefully the demand will be there this year so the water taxi will be operating as before. From Amoudi Bay a shuttle van will take you up into Oia, or you can ask someone at one of the restaurants down there to call a taxi for you. You can also walk up the steps from Amoudi Bay into Oia if exercise is what you desire.

If we are hiking on September 29 would not the crowds be significantly
less and our ability to get a cable car up the hill and a bus/cab on
the way back to Fira a bit easier?

We were there mid-October, so crowds would likely be less than summer, but still crazy.

I would recommend the hike from Imerovigli to Fira. We hiked this cliffside path in 2019 and it was a beautiful hike, just the right distance for us and in this section you walk downhill. One possibility is to take the bus from Fira to Oia. Then walk on the pedestrian streets in Oia to enjoy the specatular views, walk down to see the castle ruins and the lovely old windmills. Then you could walk back to the main square Panagia Platsani church and go down from there to the Oia bus stop. Take the bus going from Fira to Oia and when you get on ask the driver to let you off at Imerovigli. You walk from the tourist info shop 50 meters and turn right to the walking path. The breeze and views are great. You will get to see the famous Virgin Mary Orthodox church with the blue dome and 3 bells in Firostefani and continue on to Fira. Then you will be in Fira to get back to your cruise ship. I have not been on a cruise, so it would be good to check the timing and logistics to see if this is a possibility for you.

Several years ago we spent 3 nights in Oia but didn't do the hike you're considering. If you like hiking then go for it but you don't need to hike the trail to experience the views if you simply visit Oia and Fira and wander around. When you cruise into the caldera you'll get a great view of the towns ringing it above you. As an alternative, you could rent a car (driving on the island is easy) and drive to Oia stopping along the way at selected places that interest you. This will also allow you time to explore other interesting parts of the island like Akrotiri, the Red Beach, Perissa, etc. It's a small island so you can cover a lot of ground in a full day and not feel rushed.

organizer8 - thank-you for a compromise idea. Don't have to do the whole trail and leaves plenty of time for exploring Oia. We have all day - cruise does not depart until 2am next day. Appreciate response from someone who has actually hiked the trail.

We stayed in Oia and so did the hike from Oia to Fira and took the bus back. We left Oia at 10 am and and arrived in Fira at 1 pm. It took us that long because of all the pictures we took.

It was the most beautiful hike we have ever done.

Now we had two full days in Santorini but this was our favorite activity. The trail itself was not crowded until you got into towns.

But the idea of doing part of the hike may be a good one with more limited time. That part of the trail is a side walk if I remember correctly and isn't difficult at all. We liked the rougher part part because very few people.

We were in our late 50s and did it with our twenty something children.