Set parental controls on an Admin account
No - the admin user is the way that the system detects which accounts can modify a parental restriction either once or permanently.

You'll need to not use or (have) an admin account or come up with some other social contact like setting up a recurring alarm that you voluntarily obey when the time comes to cease computing.

Scheduling an automatic shut down would also work to give you some notice that it's time to be done and the system would take action unless you over ride the power down.

Just head to the energy saver preference pane and establish a schedule for the system to shut itself on and off would be my recommendation for a user with admin privileges yet wanting a reminder to keep certain hours.