I spent 15 days in Iceland (add 2 days for flights) in June, 2021.
Our Westfjord itinerary went like this:
Wake up in Skagastrond after sleeping at Salthus Guesthouse.
Drive to Hvitserkur basalt island.
Stop at KIDKA Wool Factory shop for a wool poncho souvenir
Stop in Holmavik for lunch.
Sleep at Heydalur Farm
Wake up, Go kayaking in the fjjord.
Drive to Sudavik and see the Arctic Fox Center including interacting with a rescued kit.
Drive to Isafjordur for lunch and a look around.
Sleep at Holt Inn
Visit Dynjandi Waterfall
See Bildudal, have lunch.
Sleep in Patreksfjordur
Visit Latrabjarg cliffs and puffins
Catch the ferry at Brjanslaekur
Sleep at Stykkisholmur
So, we had 3 full days.
It really depends on what your objectives are. We also went counter clockwise and this was toward the end of our trip.
My knees were done hiking by then. The scenery and views are indescribable and I am very glad we went to all of the places, but I think one day less would have been fine. We didn't really have enough to do to keep us busy for the 3 days, so be sure to have some hiking or other activities planned.
We rented a regular Subaru Outback and it was plenty sturdy for the job.
Be prepared for some minor scary driving on narrow dirt roads with no guardrails and steep drop offs. We met many large campers and semi trucks coming the other way and my nerves couldn't take it anymore. My traveling companion had to do all of the driving until we got back to paved roads.
Also be prepared for having limited food options. We were there during the pandemic and there weren't many tourists in Iceland let alone in the Westfjords. We had one pretty terrible eating experience. Another day we had dinner at a convenience store like place. We had grilled ham and cheese sandwiches and some chips/snack food.
Iceland is beautiful and like no other place on Earth. Have a great trip!