For me right now I write anytime I can. I work for my father answering phones, so I write when I'm at work. And I'm taking a philosophy class, so I write during philosophy class. (We'll see tonight how that effected my grades because we get our first in-class essay back.)
When fall semester starts, the writing I'm going to be doing is critical analysis oriented for my literature classes (I'm taking 3), and those I write at the last minute, usually starting at 10 at night and finishing around 12 or 1 the day they're due.
There's something about procrastination and pressure that gets the juices flowing.