We will fly into Holguin and out of Havana in December, so will travel up the island. We don’t want to sit on a beach. We are happy to look at old towns and nature. We will travel independently. This is part of a bigger trip Central American trip.
Beyond Havana, what are the most interesting places to see?
Looking at some blogs, we see that people spend up to a month here, but most tours just go to Havana and a few other places.
We would appreciate input to help us decide how long to allocate to this.
Thank you

If you just want to visit the major cities and tourist attractions in Cuba - 3-4 days is enough. To explore the culture and food scene (recommended) plan at least a week long trip.

Since you are apparently traveling independently-- I'd suggest you check out some itineraries of tour companies featuring trips to Cuba. In doing that you'd get a sense as to where to go beyond the two places that you initially mentioned.

I've been to Cuba 3Xs with a P-to-P and feel that I only got a slight taste of Cuban culture and its people. I want to return and spend at least 2 weeks touring the entire country. ...would have gotten that experience in 2017, but on day 3 I experienced some excruciating back pains and decided to return to the States. In looking back, I should have consulted a physician and taken this wonderful tour which was Cuban lead and based. I was traveling legal, too!

[...3 - 4 days are NOT enough. One could spend those days in Habana and still NOT have enough time to reap the intended experience of this travel.]

more than 10 days are unnecessary, just need like 3 days to meet some beaches, 5 days to meet all the interesting and cultural places and a couple of days to rest and planning

We stayed 3 weeks and didn't see the south. Another week would have been ideal.
the region i-of Pinar del Rio was for us the most spectacular. Then we loved the small cities, like Cienfuegos, Camaguey. Trinidab is beautiful. and we stayed 3-4 days at the beach to relax at the end of the trip, and visited some places with flamengos etc. Havana needs at least 3_4 days for sure.
Santiago de Cuba is said to be spectacular, but we didn't go.
We avoided Varadero that the locasl described as endless resorts...