Found this over on blogdex: Caring For Your Introvert.

Extroverts are energized by people, and wilt or fade when alone. They often seem bored by themselves, in both senses of the expression. Leave an extrovert alone for two minutes and he will reach for his cell phone. In contrast, after an hour or two of being socially "on," we introverts need to turn off and recharge. My own formula is roughly two hours alone for every hour of socializing.

For me, I'd say the ratio is about 1.5:1. I'm borderline between extrovert and introvert, according to the Myers Briggs personality test, I'm an [E,I]NFP (descriptions: ENFP, INFP).