In college it's very hard to eat enough food all day. Shakes save my a$$ when I'm out of the house . My only rule of thumb is that I don't like to consume shakes back to back, it just makes me feel odd(no stomach problems, just like I need good food). Even if I have a shake before I work out, I'll bypass a shake post workout and eat regular food.
I usually do a shake pre and one post workout, then One or two during the day, and stick with whole foods for the rest of the day. You can do protein bars if you don't want to drink so many shakes. The bars make me drink a ton of water afterwards.