How much time should be spent on learning an opening? I know this can be a subjective question depending on the opening, but in general, should on spend time on a few openings at once or should one concentrate on one opening for a while and play it every chance they get?  In general, how long should one spend on an opening, including the variations?
You're right about this being a very subjective question. I've read a few books (sorry, can't think of good references) that suggest choosing a very limited set of openings to begin with. Use this exhaustively in every situation you play, whether appropriate or not. As you attack and defend in a variety of situations you will begin to learn the merits of the position in greater depth than simply reading about them. 

As for how long to stick with it, my best suggestion (and this is indeed subjective) is to stick with the opening until you stop being surprised. Once you feel you've "seen it all", you're ready to branch out.

When you begin to spread to other openings, you may find it easier to use openings that can be transposed to from the openings you already know. The similarities will help you adjust more quickly and give you greater flexibility over the board.