This question is critical to how hard you will work between now and then. The then, of course, is the moment in which you close one chapter and begin another. It's scary to discern a shift in job but exhilirating at the same time.
A good friend who is also a Catholic priest, was just missioned to a rural part of Florida. After six wonderful years as pastor in Alabama, he was called to begin a new chapter in his ministry. In his email (i.e. "I'm moving") to family and friends, he recalled the many blessings of his work in Alabama.
More touching was his use of the famous Archbishop Oscar Romero quote which I have included below:
"It helps now and then to step back and take the long view. The kingdom is not only beyond our effoerts, it is even beyond our vision. We accomplish in our lifetime only a tiny fraction of the magnificent enterprise that is the Lord's work. Nothing we do is complete, which is another way of saying that the kingdom always lies beyond us. No statement says all that should be said. No prayer fully expresses our faith. No confession makes us perfect; no pastoral visit brings wholeness. No program accomplishes fully the church's mission. No set of goals and objective includes everything. This is who we are. We plant the seeds that some day will grow. We water seeds that were already planted, knowing that they hold future promis. We lay foundations that will need much further development. We provide yeast that produces effects far beyond our capabilities. We cannot do everything and, realizing that, there is a sense of liberation in our very being. This enables us to do something and to do it very well. It may be incomplete, but it is the difference between the master builder and the laborer. We are laborers, not master builders, ministers, not the Messiah. We are prophets of a future that is not our own."
So how long should you stay at your current job? Just long enough to make a difference and just short of getting stale on the job. Depending on who you are, this could mean a tenure of 20 months or 20 years. Therein lies the blessing and the need for discernment once a change is needed.