We do a series of sprints to build our product, then we do 4-8 weeks of hardening sprints to really test our code and get the bugs out before we deploy it in production.
(Diagram adapted from http://en.wikipedia.org/wiki/File:Waterfall_model.svg)
The problem with “hardening sprints” is that you are lying. You make believe your imaginary burndown during the initial sprints shows that you are approaching Done. But it’s a lie–you aren’t getting any closer to being ready for Production until you begin your Test phase. You wrote a pile of code that you didn’t test adequately. You don’t know how good it is, you don’t know how much work you have left to do, and you don’t know how much longer it will take, until you are deep into your Test phase.