Why its good to try a likeness sculpt – ZBrush Anatomy and Character Creation Course
Likeness sculpts or trying to copy a face from just reference images is probably the hardest thing you can do as a ZBrush artist, how ever I believe what ever level you are you should keep trying this why???
Well simply put it forces you to pay a lot of attention to your reference this does two things make you a better artist and also trains your eyes to work from images, there is of course the technical part working our which focal length the image has been shot in.
All in all i think the best way to speed up your skills is to challenge your self every day spend an hours doing a new likeness sculpt after seven days compare the result from day one I think toy will be surprised.
I also suggest starting the new week and redoing the likenesses from the previous week after that weeks up compare again and repeat by forcing you to take on these challenges you will naturally get better faster!