My brotha from anotha motha! whazaaaa?
Ok OK I'll start with the life studies, the egss they are pretty good, though some more dark shading on them would do the trick to emphasize, that art course your doing, they are teaching you to draw with lines? lines do not exist, they are created by contour of two different values/shades/colors basically you are supposed to use lines only for the sketch.
I had the best art teacher back where I came from before I moved to the US he thought me to draw with the Russian Academic way when you hold your pencil in the weirdest way not like you hold a pen but more like a brush when you are applying texture at first it was weird but after I understood it the results were amazing, are they teaching you this?
Now the Alien chick, hmm as cristina mantioned the anatomy is not quite there, the breasts are too low (is she that old?) and the hips are too fat, the ski mask doesn't add much to the whole thing as well, did you ever go through DanielC sketchbook? http://www.conceptart.org/forums/sho...d.php?t=123713 wicked stuff I dropped my jaw somewhere and couldn't find it after I reach to the middle of his SB, long story short he threw some tips and one of them is to think about the design, first choose a theme,with this one you are going for a head hunter, so is she supposed to be stealth is she hunting at night and thats why the ski mask? or is she just likes to do some snowboarding between slayings? her pose...is she modeling in between as well? either way you should put more thought in your designs and try to think about these little details that way your characters will have meaning and the things you add to them will make sense to the viewer, even though many time the viewer will not be aware of these things but it still will make visual sense to them, I think that's the difference between a good design from a brain fart.
Now regarding the time restriction I would not put any boundaries on yourself at this point of your progress, I came to realize myself that I need first to examine my work and give it a 110% and know exactly what I'm doing before I can tell myself OK you have hour and half for this so I gotta lose this and compensate there, and right now I cannot do it yet.. you know what I'm saying?
here's a small illustration of the way I see things. (Just my humble opinion)
The face study from 2 am is good!
P.S. Sorry if this Illustration will scare people away from your sketchbook I take full responsibility for this.