It's not usual practice to prune these at all - they will, though, tolerate hard pruning in mid winter, when they're dormant. Remove suckers from the base at any time.

It's true that they rarely need pruning. Here in Arizona, when they do need it, we prune lightly in early spring--about the first week of February--for shape. When the tree is about 20 years old, we will prune heavily in mid spring--mid March, about--to rejuvenate the tree, sacrificing a couple of years of fruit, to gain another 10-20 years of bearing.

Ooh, hold on, my answer was about Tilia (lime tree), not a citrus one - I have a feeling Tugbrethil is referring to the citrus fruit, so am not sure now which tree mean. Guess you have the answers to both anyway though.

Yep, I was thinking of the citrus! Tilias are rare here--in fact, I haven't seen any, though some are supposed to grow here.