Having just gotten back from Iceland, here's my recommendations:
- Reykjavik, Blue Lagoon, and Golden Circle: 2-4 days is good, 3-5 days if you want a slower pace. For the Golden Circle you might wanna consider doing a it as a part a tour instead of driving it yourself
- Reykjavik, Blue Lagoon, Golden Circle, Glacier and Black Sand Beach: 3-5 days is good, 4-6 days if you want a lower pace
- Ring road excluding Westfjords: 5-7 days is good, 6-8 days for a slower pace
- Ring road including Westfjords: 7-9 days is good, 8-10 days for a slower pace. Westfjords might not be that big geographically, but the roads are winding and go up and down so you can't pick up speed
- Everything (ring road, plus interior): 10-15 days depending on how deep interior you want explore. You'll also need a 4x4 if you want to go to the interiors
To give you a sense of where all the major things are located: