We have several employees, exempt but not management, who are required to perform approx 300 hrs/yr of additional work. These employees are required to bill 8 hr/day. These 300hrs are above and beyond the required billable hours. On top of this, if the employee is over 5 yrs, they are given three weeks vacation, but have to bill to make up the third week lost time. Does anyone have any knowledge of what the law is in this regard?

If the employees are truely exempt, then you can require that they work 24/7 every day they are not on vacation. Again, the term "Exempt" means that they are exempt from the protection of the labor laws. As such, whatever rules the employer requires will be allowed. It is persumed that exempt employees have sufficient bargaining power with the employer to negotiate fair terms for employment.

Of course, you must comply with certain rules to maintin the exemption, such as paying a proper salary. However, once they are exempt, you can work them as much as possible.