How many mg a day of Ginko Biloba?

It depends on what form the herbal is in. Only a standardized extract can provide a "dose" because that's the only time you know there is a consistent level of a specific compound available.

Generally, start with the label amount and take that for at least a few days. Then, you may choose to increase the amount by some "step" that you determine. In all fields of medicine this is simply known as "titration." You increase the amount until you get a body response. With some herbals and all drugs, titration is something that requires care because some herbals and all drugs are toxic.

But you've no doubt titrated on your own before. Ever had a "bad" headache and took three aspirin instead of two?

Ginko is considered to be generally safe, but the phytochemicals in it increase circulation in the head and neck. Because of this, too much may cause a headache.

If you're taking Ginko for "memory" or "student performance," keep in mind that it will only help if you have a known circulation problem above the head. Ginko does not improve memory, but by increasing blood flow to the brain (if it was restricted for some reason), it does therefore increase oxygen to the brain.