Assuming you are an adult (for the sake of speaking about an "adult dose"),

The dose can initially be 2.5 mg twice daily. This is increased by intervals of 2.5-5 mg weekly.
The maximum dose possible being 10 mg twice a day (20 mg daily) Higher than that is considered too high.
The general effective dose varying from person to person, from 10-20 mg being the most effective.
Keeping in mind that with other factors such as illness interactions (you can check, some of them being- glaucoma, liver disease, hypertension, psychiatric illnesses such as bipolar disorder, seizure disorders and substance abuse) and drug interactions such as Zoloft, Prozac, Trintillex and Lexapro.

Unfortunately, tolerance of stimulants tends to build quickly, particularly if we use them often, such us once or multiple times daily. This is something that can be more frequent with some stimulants, and not others, depending on the person.