Question: "Do we have an appointed time of death?"
Answer: The Bible tells us that "all the days ordained for me were written in your book before one of them came to be" (Psalm 139:16). So, yes, God knows exactly when, where, and how we will die. God knows absolutely everything about us (Psalm 139:1-6). So does this mean our fate is sealed? Does this mean we have absolutely no control over when we will die? The answer is both yes and no, depending on the perspective.
The answer is "yes" from God's perspective because God is omniscient—He knows everything and knows exactly when, where, and how we will die. Nothing we can do will change what God already knows will happen. The answer is "no" from our perspective because we do have an impact on when, where, and how we die. Obviously, a person who commits suicide causes his own death. A person who commits suicide would have lived longer had he not committed suicide. Similarly, a person who dies because of a foolish decision (e.g., drug use) "expedites" his own death. A person who dies of lung cancer from smoking would not have died in the same way or at the same time if he had not smoked. A person who dies of a heart attack due to a lifetime of extremely unhealthy eating and little exercise would not have died in the same way or at the same time if he had eaten healthier foods and exercised more. Yes, our own decisions have an undeniable impact on the manner, timing, and place of our death.
How does this affect our lives practically? We are to live each day for God. James 4:13-15 teaches us, "Now listen, you who say, 'Today or tomorrow we will go to this or that city, spend a year there, carry on business and make money.' Why, you do not even know what will happen tomorrow. What is your life? You are a mist that appears for a little while and then vanishes. Instead, you ought to say, 'If it is the Lord's will, we will live and do this or that.'" We are to make wise decisions about how we live our lives and how we take care of ourselves. And ultimately, we trust God that He is sovereign and in control of all things.