You can typically get about 1 tablespoon of zest from one medium-sized lemon and about 1-2 teaspoons of zest from one medium-sized lime.
The amount of zest you get from a lemon or lime depends on the size and freshness of the fruit, as well as how finely you zest it. A medium-sized lemon usually yields around 1 tablespoon (3 teaspoons) of zest. Limes, being smaller, typically yield about 1 to 2 teaspoons of zest. To maximize the zest you obtain, use a microplane or fine grater and avoid grating the white pith underneath the colored skin, as it is bitter. Gently roll the fruit on the counter before zesting to help release the oils and make zesting easier. Remember that the zest contains the fruit's essential oils, which provide the most intense citrus flavor.
Zest citrus fruits before juicing them. It's much easier to zest a firm, whole fruit than a squeezed one. You can also freeze leftover zest in an airtight container for later use; it retains its flavor well.