The short answer
3 days
Reykjavík and one day trip. A stopover, not the whole island.
5 days, the sweet spot
Reykjavík, Golden Circle, South Coast and a weather buffer.
7 days
Add Snæfellsnes or take the South Coast slowly. Skip the rushed Ring Road.
10 days
Ring Road with breathing room, or fewer regions at a slower pace.
14 days
Ring Road plus Westfjords, or a slower trip with rest days.