* Their retirement plan: Different retirement plans (e.g., state pension, 403(b) plans) have different rules about working and the impact on benefits. Some might have no restrictions, some might have limits on earnings before benefits are reduced, and some might have restrictions on the *type* of work (e.g., working for the same school district).
* Their health and personal preference: A retired teacher might choose to work only a few hours a week, or they might opt for a more substantial part-time or even full-time position, depending on their health, energy levels, and desire to remain active.
* School district policies: Even if their retirement plan allows it, the school district might have restrictions on hiring retirees.
In short, to know how many hours a *specific* retired teacher who retired in 2008 can work, you'd need to know the details of their individual retirement plan and any applicable school district policies.