I currently work a 12 hour work schedule (3 and 4 day work weeks).
Lately my company has instituted mandatory additional work days
to cover fellow employees who are on pregnancy leave or vacations.
Am I required to work 12 hours my normal shift or can I just work 8 hours
which is considered a normal shift in california.
I get very tired after 5 and 6 days at 12 hours per day.

In most occupations, there is no restriction against mandatory overtime. As long as the employer pays you for the overtime, they can require that you work it.

Please note there are certain restrictions for various types of work provided by other regulatory agencies -- for instance for truck drivers, airplane pilots, etc. Also California workers for AmeirCorps can not be required to work over 8 hours in a day. However, without a specific regulation, the general rule is that the employer can institute mandatory overtime.

is an exempt employee held to the same standard, since they aren't generally paid OT at all?

It is important to know that "exempt" means "exempt from the protection of the labor laws." If there are no wage and hour laws to protect you, then the employer can do anything. Thus, if you are "exempt," the employer can require that you work any amount of hours and take disciplinary action if you do not.

The theory is that "exempt" employees have enough bargaining skills and advantage to negotiate fair terms with the employer, while blue collar and other non-exempt employees would not. This is certainly not always the case in practice, but that is what the law is.