Tough question to answer as there are different varieties and sizes of lemons, and within that, some may be juicier than others. I use limes personally. A large juicy lime will give me a full serving, while a smaller variety might require 2 limes. Also, I juice large numbers of limes at one time, and freeze the juice in small ice cube trays (I will likely be scorned for this). In the trays I used, 2 cubes = 1 serving. It is more convenient in the long run and prevents frequent trips to pick up more limes.