Board members can serve any length they need to. However, you must follow your by-laws.
Boards can create internal policies guiding whether or not someone can hold a position more than once, if they must wait in between terms, if they can be president more than once, the requirements to qualify as a student, etc.
We strongly suggest not being too restrictive. Some Chapters/SIGs have a small leadership pool which is ok, but can make it difficult to fill all positions. You don't want to create policies that necessarily exclude your best people.
Create your definition of "student" for the purpose of the ballot (ACBS says that a "student" is someone who does not get their terminal degree confired sooner than half way through their term. Someone set to receive their Ph.D. (for example) 3 months into a 1 year position, would not qualify to run. Your definition may be a little different.) and determine of someone can hold a position more than once (and if consecutively), and then continue with minor changes to that document as needed (with a Board vote) over the years.