You don't need to prune them unless they are straggly or have branches growing in the wrong place (over a path etc). Or if you want to rejuvenate an old bush. As soon as they have finished flowering, cut back however far you want to. Large shrubs can even be cut to within a foot of the ground and will resprout vigorously. Bear in mind that you will not get any flowers until the new shoots are at least 2 years old. Maybe just prune back a third of the stems each year to spread the job and not sacrifice all the flowers. It depends on the reason why you want to prune.

If it's a particularly gawky variety, prune it just above where one year's growth ended, and another's began--usually visible as a ring of scars left by the terminal bud scales. Pruning there will encourage several new shoots to form, while pruning in the middle of the span often results in just one shoot, and no relief from the sparsity.