The usual length of term for a chair is three years; in small departments, it is not uncommon to take on a second term. Otherwise, there should be other people who can learn to do this, and it is good to share the responsibility. It is the Dean who appoints department chairs, in consultation with members of the department. If you know you want someone else in the job at the end of your term-or if you are in the middle of a departmental project that inclines you to stay longer-do initiate a conversation with the Dean.
Chairing a department is a demanding and difficult job, sometimes rewarding, sometimes not. If you find that it is really not something you are up to, talk to the Dean. On the other hand, if you find that you really like the administrative aspects of the job, mention that to the Dean also-there may be other ways that the college can utilize these interests.
When you know that you're in the last year of your term as chair, think about ways you can help to facilitate the transition to, and for, a new chair, especially if the person has not chaired the department before. You might invite the prospective chair to observe or participate in one or another key task: for example, planning out the course schedule for the next year (an especially important thing for the incoming chair to be closely familiar with). Talk with them about things that are on the horizon that will need to be dealt with on their watch rather than yours. Maybe go through this Guide with them, as a way of prompting discussion of one or another issue especially relevant to your department. Spend some time getting the files in order, so it's easy for the next person to find things.