Composition: work solely in black and white to break down places in an image and work with fore, mid and backgrounds. No more than 5 -10 minutes per frame. Ideally much less than that. Start with a light 'gradient' in the frame.
Light and colour in environment: working with colour from the start, trying to figure out how palettes work - often one image relies on a low number of overall colors. Usually around 30mins to an hour each. I tend to either google these images or browse dA's photography sections.
Faces/people: a mix of greyscale and colour. usually any number of reasons for studying - anatomy, light, form, colour. There are literally an infinite number of possibilities with faces. If I'm looking just at form I usually study busts as there's no colour to confuse. If i'm looking at overall body I often resort to lineowrk as it's much quicker and achievable in my lunch hour. These range from about 1-2hours for the colour/black and white, and 30 mins or so for each linework study. Sources include Conceptart's paint from what does everyone look like thread (in the lounge), dA's stock section and googling for street fashion.