Invest in Nathan Vegdahl’s Humane Rigging. It’s not a matter of learning face rigging, it’s about learning rigging (full stop). Study that, learn it well, and you’ll find very little to stump you in the rigging arena.
When I first looked at rigging, I thought I could knock it off in a couple of months. After all, I learned modeling in that space of time and texturing in a couple more months. But rigging (and I don’t think this is peculiar to Blender) consists of a few very basic ideas that, when you start layering and combining them, become very complex, very quickly. You have to understand the underlying ideas and how they work together. Then you need to plan your rigs. With the right knowledge (which Nathan imparts) you’ll be able to predict most rigging problems and fix them.
Give yourself at least a year to understand rigging.