We spent a month in Istanbul this summer. And it wasn't enough. )))
I should say that the city is fantastic, there's so much to see and explore. People are kind, gardens and parks are beautiful, a lot of architecture of any kind from ancient buildings to modern skyscrapers. There's even some beaches available. FYI Istanbul is number one touristic destination in the world according to Tripadvisor.
I've been to Prague, Paris, Moscow. Istanbul wins, just go there. Spring and autumn are the best seasons to visit the city. We were there in July, so it was a little hot, even for natives ))