First time in Istanbul: how many days and which neighborhoods?

Three full days is a solid minimum. Stay near Sultanahmet for the major sites (Hagia Sophia, Blue Mosque), then spend an afternoon in Karaköy and Galata for cafés and views.

If you’re into food, spend a night in Kadıköy on the Asian side. It’s lively, cheaper, and the meze spots are great.

I’d do 4–5 days if you can. One day for the historical core, one for markets (Spice Bazaar + Grand Bazaar), and one for the Bosphorus cruise + neighborhoods.

Stay in Karaköy if you want to walk everywhere without feeling stuck in tourist central. Easy access to ferries and great coffee shops.

Tip: get an Istanbulkart. It makes transit simple and saves money on ferries and trams.