5-6 days would be a good start for Havana and surrounds. There is plenty of nightlife, many places to stroll and absorb the atmosphere, and beaches within 20 minutes travel. Cuba, IMO is not about rushing around to see sights, but lingering to soak up the culture and converse with local people.
f you want to travel to the countryside (Vianles), other historic cities (Trinidade or Matanzas, for example) or one of the nicer beaches (Veradero or one of the cayos), add some time.

Since you are from the US, you will probably use the OFAC reason for travel "Support of the Cuban People" as most independent travelers currently do. Choosing this reason implies that you will spend money supporting independent private Cuban businesses, including casas particulares, as well as paladares. private taxis and tour guides. In addition, a casa particular is a great opportunity to meet some Cubans and get a bit of understanding of their lives.
Here is a list of Cuban agencies (not US based ones like AirBnB) where 100% of the money stays with Cubans. Some let you look at photos and reviews and choose; others ask for your preferences and then suggest casas that might fit. Casas currently run the gamut from a comfortable, air conditioned room in someone's apartment to a whole mansion with chic decor, with corresponding pricing. I have personally had good luck with Cuba Junky.
Casa Agencies
http://www.tour4cuba.com
http://www.casasdealquilerencuba.com
http://www.alquilerencuba.com
Cuba Vacation Rental ? Homes, Apartments, Beach Houses & Rooms for Rent in Cuba
Websites to help find Casas in Cuba - Cuba - Debbie's Carribean Reviews Forums
Cuba-Junky - All About Cuba Traveler's information and Reviews
http://www.casasdecuba-en.net/
casa particular cuba .org - Casa Particular organization for renting private rooms in Havana and all Cuba
http://havanacasaparticular.com/
CasaParticular.org - Organization of casas particulares in Cuba - Online Bookings
https://www.mycasaparticular.com/en/
http://www.cubaccommodation.com/
bedincuba.com
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http://www.casaparticular.com
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http://yourcasaparticular.com/
Casa Particular Cuba 2018 ~ Stay in a Casa Particular Havana and all Cuba with CubaParticular.
www.bbinnvinales.com
[email protected] (Dalia)
www.Cubabookingroom.com
www.cubacasas.net