This is a podcast with Seth Godin being interviewed by John Jantsch of www.ducttapemarketing.com that many will find interesting.  Seth is one of the early Internet entrepreneurs who is also an author of a new book called "The Dip" A little book that teaches you when to quit and when to stick.  Seth talks about how if you are still seeing progress in your business it is not time to quit, however if you are not gaining any measuarable momentum it is time to quit.

http://www.johnjantsch.com/podcasts/seth.mp3