Super fun and exotic city with a wide mix of many cultures (European, Asian, Arabic, probably other stuff too).
Istanbul and Turkish people/culture are such a trip. For me being an American with lots of experience in Western Europe and South America, Turkey was really exotic, energetic, fiery fiesty. The Istanbul vibe is something right in between a big cold metropolitan city like New York or Paris but with the feistiness of South Americans. Again, they are not outwardly warm and friendly but are feisty.
Maybe you expected a hyper-Muslim culture here and there is but it’s only part of Turkey like this where people are super religious and women all covered up in traditional headwear. Most Turkish (especially the new generation) curse, drink, party in clubs, and engage in all kinds of obnoxious “Western debauchery” like you would find anywhere else. Long story short, Istanbul is as much a party place as any other. Don’t be afraid that you’re stuck in some overly religious prison ground.
There’s enough craziness here to last you a solid month of fun but you could see the main points in 3 days. I definitely recommend 4-7 days so you can relax and not feel like you’re always stuck in an anxious taxi. The pace of life here seems fast and crazy so try to find time to slow down. If you have more time, there’s a handful other worthwhile cities in Turkey to visit as well.