Have you ever had the experience of going into a piece of code to make a seemingly simple change and then realizing that you\'ve just stepped into a wasteland tha
Starting assumption: you already "commit early, commit often".
Then there's never a bad time to start a refactoring, because you can easily back out at any point. I find that at the beginning, it always feels like it's going to be easy, but after making a few changes and seeing the effects, I start to realise quite how big a job it's going to be. That is the time to ask whether there is really time to make the changes, or whether living with it until next time you come to this code is the better course.
The willingness to stop, throw away a half-refactored branch, and do it the nasty but quick way where appropriate, is key.
That's for refactoring, where the changes are incremental, and running (or nearly-running) software keeps you well grounded. Rewriting is different, because the time until you figure out it's going to take longer than you thought is so much greater. I may be taking the question too literally, but there's almost never a good time to throw it away and start again.