It helps to understand that the word "missionary" does not appear in the Bible. It is not a church office, and God does not promise to give missionaries to the church. God's Word sets no standards for missionaries, and so there is no Scriptural regulation for this office.
I was a missionary to Massachusetts one summer while I was in college. I spent forty hours a week knocking on doors, plus I ran a bus route. We have a teenager down here now who has lived in orphanages for the past few years, working without pay. Someone else just posted a comment about how she led her first soul to Christ on a mission trip.
And others go on a working vacation, doing less work than they would on a summer job. When they report back to their church, they spread their distorted ideas of mission work to their listeners. A happy congregation is pleased that their young people had a good spiritual experience and misses the fact that they came back empty-handed.
This is one reason why most missionaries, mission boards, and churches that support missionaries don't know what they're doing.