Pretend that you are a Project Manager scheduling someone else's time. You make a list in priority order of everything you have been assigned. You calculate effort in days and duration in days. When someone asks you to do one more thing, you bring out your list and you schedule it in together. They will soon see that the one more thing will be finished in 7 months (too late to be useful) or another important thing will be delayed until past the time it is required. Unfortunately, the one more thing will be given to someone who is less organised and will probably not deliver something. Fortunately or unfortunately, depending on your perspective, you will be promoted to manage IT.