You know, the best metaphor I've heard, and how I've started treating it ever since: Twitter is like a cocktail party that you can enter and leave any time of the day, for as long or as short as you'd like. You might participate in some interesting conversations, you might learn about something new, and then when you're done you can leave.

The thing you don't want to do is try to keep up with every tweet. Just follow a bunch of people you find interesting, and once in a while pop in and see what people are talking about.