The studies are all over the place.  My philosophy - be happy!

Women who drink 1 or more cups a day

May be at a 25 percent lower risk for stroke

The journal Stroke

3 to 4 cups a day

Reduce Type 2 Diabetes Risk by 25%

Institute for Scientific Information on Coffee (ISIC)

4 or more cups a day

Insomnia

Nervousness

Restlessness

Irritability

Stomach upset

Fast heartbeat

Muscle tremors

Mayo Clinic

Those 55 years and under who consumed more than 28 cups of coffee a week (4 cups a day)

Are twice as likely to die compared to non-drinkers.

Aerobics Center Longitudinal Study

6 cups

Drinking up to six cups a day of coffee is not associated with increased risk of death from any cause, or death from cancer or cardiovascular disease.

Harvard