Here are some recent quick studies from life. I probably spent 30 minutes to an hour on each.
I'm really liking limiting the time spent on each of these. It forces me to think about the hierarchy of importance, and go after the most essential elements, and leave the canvas before I get bogged down with subordinate stuff.
It also helps me to say what I need to say in fewer strokes. Sometimes that means using the brush strokes gesturely, and other times it means just simplifying complex areas.
Limiting how much time you have to paint may encourage one to paint faster by using rapid strokes, but I think this is a mistake. It tends to result in sloppy, thoughtless sketches. I'm guilty of it myself, and my impatience shows when I get into that rapid brush mode.
So what do you do if you have limited time? I think it's far better to gain speed not by faster strokes, but by using fewer strokes. Each stroke still need to be executed with care. In fact, more so because you have to say more with fewer strokes.
What if you allow yourself a limited number of strokes, instead of limited time? And when you have reached your limit, just stop, and move onto a fresh canvas and start again. If you didn't get far with the first one, you know you need to economize with the second, and so on.
Just how many strokes is enough for a head, depends on many things; how you paint, the size of the head on the canvas, the size of the brush, how much paint is on your brush, etc... You ultimately have only your own work to compare with, so I'd suggest painting a head as you normally would, counting your stroke. If you find that you used 500 strokes, try to do the next one in 250. If you're successful, perhaps you can try the next one in 125. You may or may not like the sketches you've produced, but I guarantee that your strokes will be more thoughtful and intentional, and that will undoubtedly improve all your paintings, not just the limited time/stroke studies.
Another way to simplify your sketches - and thereby lessening the time it takes to execute it – is to limit your palette. This one, as well as a few of the others in this post was done with the Zorn Palette; Cad Red, Yellow Ochre, Black, and White. Plus a transparent brown to tone the canvas.
Or how about limiting the number of distinct values? You only need two values to describe form, and you can add third and forth values if you have time. It's a logical way to simplify something complex. The idea being, you don't have to guess at - and be confused by - the task of simplification.
So go enjoy some quickies~