I just read an article in this April's Kiplinger's Personal Finance, Do you talk too much? by Marty Nemko. In fact, here's a slightly altered version of the article on Marty's site [probably due to copyright/publishing issues with Kiplinger's].

There is some good advice in the article, but the main thing that caught my attention was the traffic-light rule of thumb.

During the first 30 seconds, the light is green and your listener probably isn't bored. During the next 30 seconds, the light is yellow - your risk of annoying the listener increases. Look for a place to stop. After 60 seconds, the light is red. There may be rare times when you should run a red light - for example, when you're sharing a fascinating anecdote - but, usually, you'd better stop.

Marty also gives an interesting Fran Lebowitz quote: "There is no listening. There's just waiting for the other person to stop talking."

This is why I like writing/blogging. If you don't like what I'm saying [writing] you can click on that little X up in the top right corner to make me shut up...and my feelings won't be hurt. 

~tod