Truth is many of the guidelines for the use of ginger in preventing nausea, vomiting & seasickness come from herbal lore, although one pretty solid study found 1 gram/1,000 mg of powdered ginger to do the trick.
I found sources that variously recommend anywhere from 1/2 gram/500 mg to as much as 4 grams/4,000 mg.
As with any drug or supplement, one is generally better off using the smallest effective dose. This may involve some trial & error.
Ginger in powdered, root or crystallized form is probably the easiest to control in terms of the amount of active ingredient ingested. Eating ginger snaps, or drinking ginger soda or tea makes control more difficult. And be aware that some products contain only ginger flavoring not real ginger--read the labels.
As for timing, estimates range from several hours to 20 minutes prior to departing, depending on the source. The matter really hasn't been studied sufficiently to say with any precision. The same issue pertains to frequency of dosage. The limited studies involving repeated doses used intervals of 4-6 hours.
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual, and should not be construed as such.
Best regards.
DocVikingo