Never.
When I was on Staff, I used to go in there for about an hour or so whenever I was online, because that was part of our job, but I was never very active, I just looked out for the same stuff I looked out for on the boards and dealt with what needed to be dealt with. It was never something that I actively wanted to be doing.
Like Kean, when I'm online, I tend to be doing a million things at once, and I just don't have the attention span that it takes to be actively involved in the chat. I'll get distracted by fic or art, and I'll lose the whole grasp of the conversation.
Back in the old days, when we had a million people in there at once, things moved way too fast for me to grasp the conversation, and I would be too slow to keep up and I'd get annoyed if I missed something.
That's the main reason I won't take part in a chat based RPG, because things move to fast for me, and I'm scared I might miss something important for my character or integral to the plot.