next time. For a project that lasted a year, you'll want to spend more time. Short changing your retrospective means short changing your chance to do better next time. Improvement doesn't happen by hoping; teams need to dedicate time thinking and learning.
Successful organizations know how to evolve to meet ever-changing expectations, rather than holding onto what used to work. Becoming comfortable with change, learning how to try something new, and measuring how well it works are critical business skills. And retrospectives are a great way to learn those skills on a team level.