The short answer: four days is comfortable, three is enough, two is tight but possible.
Two days — the essentials
Day 1: Hagia Sophia, Blue Mosque, Hippodrome, Basilica Cistern, Grand Bazaar.
Day 2: Topkapı Palace, Spice Bazaar, Bosphorus cruise.
You will be tired and you will miss the Asian side entirely, but you will have seen the icons.
Three days — add breathing room
Split the old city over two days instead of one, and give day three to the Bosphorus, Ortaköy and Çamlıca Hill.
Four days — the sweet spot
Add Beyoğlu: Galata Tower, İstiklal Street, Karaköy for coffee, and Balat for the coloured houses and antique shops. This is where Istanbul stops being a monument list and starts being a city.
Seven days — Istanbul plus
Five days in the city, two for a day trip to Bursa or Sapanca, or an overnight to Cappadocia.
Things worth knowing
- Hagia Sophia and the Blue Mosque are working mosques and close during prayers, especially Friday midday.
- Topkapı Palace is closed on Tuesdays. The Harem needs a separate ticket.
- Distances look short on a map but the traffic is real. Two major sights per day is a sensible target.
- The Museum Pass pays for itself if you visit three or more state museums.
