Typically, a sprig of thyme yields between 1/4 and 1/2 teaspoon of leaves. This range occurs because a "sprig" indicates one branch or shoot of a plant, and there is no exact size for this.
Determining exactly how much thyme to use in a recipe requires tasting along the way. First, cooks should sample the leaves themselves. Older plants tend to have larger leaves, and they also have a stronger flavor. Cooks need to consider this strength when adding thyme. In a recipe that calls for one sprig, it is helpful to begin with 1/4 teaspoon of leaves. As the dish cooks, the cook can sample it to see if more thyme needed. The dish needs to be checked again before serving as flavor gets lost during the cooking process; more can be added to taste.