Philadelphus will benefit from pruning although it is not essential every year. If the shrub is getting too large, or has developed some spindly branches, or it is flowering less, prune it to promote more growth and vigour. The best time to prune Philadelphus is after flowering, which will be later in July usually, and cut back to good bud and /or remove about a quarter of the old growth. Like all shrubs regular pruning of the older growth with promote new growth and better flowering. If pruning regularly every year it is common to remove one third or a quarter of the old growth down to the ground.
Over time Philadelphus can become woody, with all the flowers at the top, and long wood/cane like stems which generally looks unattractive. Philadelphus belongs to that group of shrubs which will tolerate hard pruning and come back although it may take more than one season. If your Philadelphus shrub has become overgrown and unsightly, prune it hard, around July after flowering down to 1ft/ 30 cms. It may not flower the following year but should put on a good deal of growth and by year 2 will look a great deal better and blooming.