From personal experience, the most important thing is to do it every day. 15-20 minutes is enough. Your brain will work on technique issues overnight. The jump in skill after 30 days of daily sketching can be substantial.
With respect to drawing people, viewers respond strongly to particular parts being "wrong", so break the learning process down into parts. For example, focus on over-all proportions, then focus on hands and wrists, then on faces, and then feet. Then put all of it together.
With respect to drawing people, viewers respond strongly to particular parts being "wrong", so break the learning process down into parts. For example, focus on over-all proportions, then focus on hands and wrists, then on faces, and then feet. Then put all of it together.