The best answer I can give you is "rarely".
Deciduous trees are pruned because there may be a weak branch. The fruit on a deciduous tree such as an apple or peach can improve in size if the tree is pruned. This isn't so with citrus. Pruning can improve the quality/taste of fruit on deciduous trees-not so on citrus.
Prune citrus of dead branches, those that cross, and suckers coming from below the graft. You can also raise the skirt on the tree so the fruit is not dragging on the ground.