even an hour is good if you really practice

I think there shouldn´t be any problem with practicing forms and faces at the same time if you take it as excersise of applying those forms =) And doing only inanimate shapes over and over again can be a bit dull, so you need something to keep you interested anyways =) little advice: when you practice doing circles and other shapes, try to do as few lines as possible, it will help you tu build confidence with your lines and your drawings will be more clear later on. It´s good to practice it in the early stages than trying to get rid of mesy lines later =)