Each local church decides the issue. I can only speak for ours. We are a congregational church. Knowing our congregation, we will never elect elders. Elder is the office for representational governing authority. Deacons have no Biblical governing authority. Deacons are servents, and are there to serve the Lord and, except in unusual circumstances, to serve the pastor. That is how our church operates. Fifty or so years ago, we had a "board" of Deacons that thought they ran the church. That has gone by the wayside. Deacons can make recommendations, but the church is the deciding authority.
At the present time, we are between pastors. He was here for 13 years. He is leaving due to the care his wife now requires. We will elect a pulpit committee. The committee will search for a pastor, and bring a recommendation to the church. Probably after a sermon or so from the finalist, the church will discuss and vote on the candidate. A two thirds vote is required for a call from the church. The vote has never approached that. Our lowest vote was the pastor before the one now leaving, and he got 91%. I would think a pastor would not accept a call with a 70% vote. Most are unanimous. To answer one of the questions of the op, we put no time limit on his term.
Deacons who think they have governing authority hold an office with a total misconception of their role. As far as the subject of elders, elders are suppose to be elected on spiritual maturity, but it usually ends up being on the basis on social status. From my experience with elders in the Presbyterian Church, when this happens, they are no more mature or Biblically knowledgeable than the average active church member.