Many people ask this question, but if a person is a true believer, you are a disciple of Jesus Christ and you have surrendered to Him as Lord and you are His slave.
You want to follow Jesus for the rest of your life, whether you are in ministry overseas, or in ministry in your own country/culture, or you are a just an average Christian in a local church who shares in the missionary vision and prays for missions, but has a full-time job and family and cannot spend as much time in missions issues as you might like. Sometimes God changes our circumstances and God’s providence change our status from being overseas to coming back home to the USA or your home country/culture. If you are surrendered to Christ as Lord, don’t worry about the question, “how long do I have to commit to missions?” Don’t even ask the question. Just follow the Lord and abide in Christ, the true vine, every day. (John 15:1-16)