Pruning these should consist of cutting back flowered shoots to where a new bud or branch is forming. In practice, though I've known them cut to the ground and still grow the following year, so its up to you really.

We had an old Philadelphus some time ago. It got too large and we cut it back severely. It sulked after that.......eventually it was taken out. All gardeners have to think what to do with plants, trees or shrubs that get over large. There are so many enticing plants to take their place