Many varieties of privet hedges exist: Amur privet, Chinese privet, and Japanese privet, to name a few. These hedges, if left untrimmed, can reach a height of 16 feet. They are deciduous shrubs that retain their leaves into winter, and drop them before springtime.
In most instances, privets act as privacy screens and windbreaks. The main reason to trim privet hedges will be to maintain your desired size and shape within your landscape design. Trim the hedge from late spring through mid-autumn.
Place a stake (which is the desired height of your trimmed hedge) at each end of the hedge. Tie a string to the top each stake. Step back and make sure that the line is level and straight as this is going to be your string guideline system when cutting. (You will be trimming the top of your hedge using the string as a height - guideline when you cut.) It is entirely up to you whether you want to, or need to, set up the string as a guideline system when you trim. Some gardeners find that they can trim the hedge without a string guideline system, and they rely on their own visual sight line.
Trim your hedge to the desired height, using either an electric hedge trimmer or a manual hedge trimmer. Electric hedge trimmers are quicker and less labor intensive, but be extremely careful when using them and always wear safety glasses when using an electric hedge trimmer.
Trim the sides of the hedge to your desired width using the electric hedge trimmer or the manual hedge trimmer. Shape the sides of the hedge so that the top of the hedge is slightly narrower than the base of the hedge. This allows sunlight to reach the bottom branches of the shrub, and therefore you will not have bare branches at the base of the shrub due to lack of sunlight.
The amount of trimming you should do depends upon the desired width of the hedge. If you have been regularly trimming, and shaping your hedge then the amount that you remove will be minimal. (Although, privet hedges are resilient and can take radical pruning. Radical pruning/trimming is sometimes required for old hedges as radical pruning produces vigorous new growth.) Dispose of the plant material in your compost bin.