Address issues as they are raised. Don't wait too long to get rid of debt, and most certainly don't rely on fixed time cycles of fixing things. A team has good sense of the issues they deal with, and you can capture their view in retrospective meetings. Depending on how bad the debt is, you should allocate time to improve. Velocity will only get worse as debt grows, so there is a clear business case for such stories: the more debt, the slower the velocity and the lower the quality. Spend time now, to speed things up. But just don't do it from a theoretical perspective. Address real issues, set real goals and measure them.
