Stevia leaf extracts are a mixture of sweetening compounds called steviol glycosides. In order to establish an acceptable daily intake for the mixture, safety studies calculate intake based on the common feature in the steviol glycosides – the steviol backbone. Therefore, the acceptable daily intake for stevia leaf extract has been established and adopted globally recognized safety studies as 2mg per pound of body weight expressed in steviol equivalents. Safety authorities, such as the World Health Organization (WHO), have calculated this intake level to include a substantially high (100 fold) safety margin. You can find more information about the calculation of daily intake here.
An adult weighing 130 pounds can consume an amount of stevia leaf extract with sweetening power equivalent to about 45 teaspoons of sugar a day, however, due to their lower weight, children reach the recommended daily intake faster than adults: a child that is half as heavy can safely consume half as much (approximately 22 teaspoons).