Focus on Continuous Improvement
A key principle of Scrum is continuous improvement. The Scrum Guide says:
Scrum makes clear the relative efficacy of your product management and work techniques so that you can continuously improve the product, the team, and the working environment.
While teams will rarely see dramatic improvements once they've achieved maturity, they should continue to see refinements and changes over time as an outcome of the inspect-and-adapt cycle. Team compositions change, projects change, and business needs change. No matter how efficient the team may be today, an effective Scrum Master should never stop helping the Scrum Team look for new insights into their processes, and provide opportunities for the team to discover new and better ways of working together.
Beyond the Scrum Guide, the role of Scrum Master should evolve with the team. As a Scrum Team and its parent organization matures, the Scrum Master often becomes less of a process referee and more of an efficiency coach. Scrum Masters are members of the Scrum Team, and should be using significant slack time to explore new techniques, practices, tools, and workflow ideas to share with the team as it seeks continuous improvement.
Even within the singular role of Scrum Master, there are always opportunities for continuous improvement. Seeking out better metrics, better information radiators, improving communications and facilitation skills, and continuing education in agile techniques and thought leadership should all be evergreen tasks for a Scrum Master who is part of a mature Scrum Team.
Inspect Framework Fit as Projects and Teams Evolve
Projects are also not infinite. By definition, a project has a beginning, a middle, and an end. Mature Scrum Teams often stay together for multiple projects, so there's always implementation and adaptation work to be done as the team onboards new stakeholders, new business drivers, and new workflows. If none of these apply, and you have a never-ending "project" that is more maintenance or support than development, you may have uncovered an opportunity to explore other frameworks, educate the organization, or transfer non-development work to a different organizational unit.
Avoid the 100% Utilization Fallacy; Seek Value Instead
Roles in Scrum do not require 100% utilization to add value. If the team and the organization have truly reached total and complete enlightenment, then there is still value in having an experienced Scrum Master available for the next project. There is also value in having this person paving the way for future projects, training other members of the team or organization in the Scrum Master role to increase cross-functional expertise and prepare for scaled agile implementations, and other Scrum-related organizational improvements.
Continuously Adapt Your Career
And finally, if you've truly done every single thing the role, team, project, and company can possibly benefit from, it may be time to continuously improve your career. Your goal as an agilist shouldn't be to grow old and die in your current role or at your current company. There are always new teams, new projects, and new companies in need of a great Scrum Master. If you can't add value or continuously improve where you are, then it may be time to move on.