Inspect and Adapt
In addition to the daily 15 minutes, Scrum prescribes that the team have two meetings at the end of the sprint to inspect progress (the sprint review) and identify opportunities for process improvement (the sprint retrospective). Together, these should take about 5% of the sprint, or one day for a monthly sprint. Alistair Cockburn has described the goal of the retrospective well: "Can we deliver more easily or better?"9 Retrospectives force the team to reflect on opportunities for improvement while the experience is fresh.
Based on the retrospective, the sprint end is a good boundary at which to make process changes. You can tune based on experience, and you can adjust for context. For example, you might increase the check-in requirements for code review as your project approaches production and use TFS check-in policies, check-in notes, and build workflow to enforce these requirements.