How much notice should I give my boss for surgery? Not officially scheduled, ~6 months away.

That would vary, obviously.  Two months seems reasonable, but every situation is different.  You did not say how long you were going to be off work.  My breast augmentation patients are out for about four days, and tummy tucks are about ten, more if their work is quite physical.
You could ask your boss how much notice he would appreciate.  If I were an employer, I would like that the employee was giving me enough consideration to ask me.

The further in advance you give your notice, the better.    I recommend an in-office examination as well as a detailed discussion with a surgeon who you are comfortable with.   Finally, make sure your surgeon is a Double-Board Certified Plastic Surgeon certified by the American Board of Plastic Surgery (ABPS), a member of the American Society of Plastic Surgery (ASPS), a member of the American Society of Aesthetic Plastic Surgery (ASAPS), and a fellow of the American College of Surgeons (FACS).Best,Dr. DesaiBeverly Hills Institute for Aesthetic Plastic SurgeryHarvard Educated, Beverly Hills & Miami Beach TrainedDouble Board-Certified, Double Fellowship-Trained Beverly Hills Plastic Surgeon

Thank you for your question. I would say that your HR department is a better resource for this question than a plastic surgeon forum. I guess it depends on your company, but I would say that the more notice, the better. Get a good understanding of what the surgical recovery is going to be and the downtime and try to be open and honest with your employer. Good luck!