In South Asian cooking the Kaffir lime leaf is used more often than its fruit.
The main season for harvesting kaffir limes in Europe is from mid August to November. After that the essential oil content diminishes. The fruit turns yellow in December and January and is not less aromatic, the juice is very sour.
They keep well when frozen. Though they do suffer the a loss of aroma. The fruit can also be cut in slices and dried.
The zest has a different flavour from the leaf, it is fruity & spicy . It takes a skilled hand to remove the zest without touching the white underneath, which is very bitter.
In Thailand the zest is used for seasoning and to prepare a flavonoid rich tea, promoted as a natural anti-oxidant.The peel can also be dried preserved and candied. The zest is commonly used in Creole cuisine and to flavor rums in the French Reunion Island and Madagasgar.
You can make spicy preserves in brine using the whole fruit or just the zest. You can make jam or gin. Or you can use them to make shampoo.