Never.
Well, almost never. When you IM (or SMS, or call) somebody, they have to stop whatever they are doing and start doing whatever you are doing. If you are okay with that, you have my permission to send that chat message. If not, send an email. Here are some examples:
Can you guess when to use an email and when to use a more immediate form of communication? I bet you can. Given that it takes the average person several minutes to re-focus after they’ve been interrupted, it’s worth taking a moment to think this through. If you’re abusing IM, you’re being abusive towards the people you’re messaging.
And remember: only you can prevent collie fires.