How long does 20 mg adderall last for a 100 pound girl?

Since most people's brains are about the same size no matter what their weight, body size doesn't really matter.

It's best to start with a low dose, like 5 or 10mg per day, then increase it after a week until you start experiencing trouble sleeping, serious lack of appetite, headaches, elevated heart rate, or other troubling side effects. Then dial it back to the point where you didn't have so much trouble. The average final dose is 20mg, but that might be too much for some people and too little for others.*

After taking it for a long time, some people have to increase their dose because a tolerance builds up. However, unlike a lot of psychoactive prescription drugs, you can stop taking it on weekends or holidays when you don't really need it, without ill effects.

It should never be taken except under a doctor's supervision. It can cause heart attack, stroke, and strong effects on blood pressure. It can also cause severe anxiety, panic, insomnia, weight loss, and even psychosis.

Don't take it without a prescription or if you have an existing heart condition. It also interacts dangerously with many prescription and recreational drugs, including antidepressants, heroin and many "downers", and alcohol.

* You might want to ask your doctor for a few small prescriptions (only a few pills) at 5, 10, 15, 20mg, so you don't end up spending a lot of money on a month's worth of pills you can't use.