Everybody is different. Some people switch subjects every day, some every week and some finish an entire subject before moving on. It depends on how what you like to do. However, I've heard a lot of people that studied one subject at a time, later to go on and say that if they did it all over again, that they would alternate subjects daily. I've never heard it the other way around.
In my opinion, I think it's good to alternate subjects, seeing how they intertwine and you can always stay fresh in each subject.