5 days — One coast
Pick one area (Alghero + northwest OR Costa Smeralda + northeast OR south coast + Cagliari). Five days explores one coast well but you miss the rest.
7 days — Two coasts
Alghero (3) + Cagliari (2) + Ogliastra east coast (2). Or north to south via the interior. Seven days is the minimum for a "proper" Sardinia experience.
10-14 days — The full island
A road trip loop: Cagliari (2) → Ogliastra coast (3) → Costa Smeralda/La Maddalena (3) → Alghero (3). Beaches, mountains, Nuragic ruins, and the wild interior.
2+ weeks — Sardinian life
Add: Barumini Su Nuraxi (UNESCO), Gennargentu mountains, Bosa (colourful town), Carloforte (Ligurian-Sardinian island). Two weeks is when Sardinia stops being a holiday and starts being therapy.
Our recommendation: 7-10 days with a car
Car rental is absolutely essential in Sardinia. Book months ahead for summer — availability runs out. The SS125 Orientale Sarda coastal road is one of Italy's best drives.