It depends a lot on the product. When I worked in a field where our code had to be outside-audited it was a planned part of our sprint. PM just asked development what area needed refactoring and it was put in the plan. That's not to say you wouldn't fix the code in the area you were working on, but you wouldn't devote a day to rewriting a mangled chunk of code that worked. Now I'm working in scrum and developers just do it as they work. My impression is that about the same amount of time goes into refactoring work, either way.
