Hi Diane, it's going to depend very much on the type of shrub or bush it is. Here's a handy guide that will hopefully help you out - https://www.stihl.com/how-to-maintain-and-trim-your-hedge.aspx
As Michelle stated, it depends on the shrub. If they are flowering shrubs it is even more important to know the name of the shrub you're trimming. Some flowering shrubs bloom on last years wood and some bloom on new growth. I trim some of my shrubs in the spring and some in the fall. The flowering shrubs that I have get trimmed when they are done flowering. Try to identify each shrub and look up each one to trim it so that it looks it's best. Wishing you the best.
As said, depends on the shrub but you can take a scissors to them at random times to cut back anything that is way too long or infringing on other plantings. It also depends where you live. I'm in FLaDuh and everything grows so fast that I can trim anytime. Back up north, spring and fall are the most common times to trim.