While you could spend a week exploring the Douro Valley, two days is generally considered the "sweet spot" to experience the essential highlights of Porto. On the first day, you can focus on the historic Ribeira district (a UNESCO World Heritage site), walk across the iconic Dom Luís I Bridge, and tour the famous Port wine cellars in Vila Nova de Gaia for a tasting. On the second day, you can explore the uphill sights, such as the Clérigos Tower, the stunning azulejo tiles at São Bento Station, and the world-famous Livraria Lello bookstore. Porto is a very compact and walkable city, though its steep hills can be physically demanding. By spending 48 hours there, you have enough time to enjoy a leisurely Francesinha lunch and a sunset at the Jardim do Morro without feeling rushed. However, if you wish to take a day trip to the Douro Valley vineyards or the historic city of Braga, you would definitely need to add a third or fourth day to your itinerary. For a city-break focused purely on the urban core and the riverside, two days is perfectly sufficient to capture Porto's unique, melancholic charm.