I'm planning to travel to the Netherlands, Bulgaria and Turkey. I'll be in Netherlands for 4 days, Bulgaria for 4 and Turkey for 2. I'm applying for a Schengen visa for the Netherlands. Considering that Bulgaria and Turkey are not in the Schengen Area (but accept Schengen Visas for entry), will the duration of the visa be determined by only the Netherlands leg of the trip? Ideally I would want the visa to be valid for at least 10 days to include the entire trip.
As far as I understand as @Annoyed said Bulgaria and Turkey are not part of Schengen area right now. Even though you enter those countries, it's considered as you exit Schengen area and if you only have single-entry Schengen visa you might have a problem entering Schengen area again. I used to do this when I entered Croatia which they also accept any holders of Schengen Visa I got stamped at the border. This means that I exited Schengen area and entered Croatia.
So to answer your question since those countries are not part of Schengen visa, it will not be considered in your visa process. You might only get a single-entry-4-day Schengen visa from the embassy at the very worse case.