For a first visit 3–4 days is enough: museums and the waterfront, Souq Waqif, optionally the desert. It's a city-break, not a two-week beach resort. In practice: the best window is November–March (less heat); don't pack a tight programme on departure day; and check entry rules and stay length on the Qatar page. A day-by-day plan is in the 'Qatar: 3 days' route.