Refactor a lot, but only when it's appropriate

Don't just refactor for fun. Refactor in service of delivering business value. If there's some terrible code that you never need to touch, then there's no reason to change it. Leave it terrible.

So, when are the right times to refactor?

• When you're changing code. Refactor to make it well-designed for its new purpose.
  
• When you're reading code. Every time you gain some understanding, refactor to record that understanding. Lots of renames.
  
• When you're afraid of code. If there's code you should be changing or reading, but you avoid because it's such a mess, then you should definitely refactor it.

Note that this refactoring is a small improvement each time, not a dramatic major rewrite. The goal is Better, not Good.