It depends. This time it depends upon the species, but mostly you're safe to trim evergreens that don't flower in summer and fall. Go to the shrubs forum and read the FAQ on trimming shrubs, then read these two recent posts (and probably more):

"Evergreen? Trimming help/opinion"

"It breaks my heart but...."

They both have pics and some good advice. You could very easily have one of these two common hedges pictured.

Good luck.

Hi, I did this recently with the Azaleas in front of my house - we actually used a chainsaw to cut about a third off all of them. They seem to be leafing out again, no problem. Don't know if they'll bloom next year though.

It's generally best to prune evergreens in spring through early summer before the main growth spurt, and azaleas and flowering bushes should be pruned right after they flower. Fall is not a good time because new growth won't have a chance to harden off and is more susceptible to winter damage. Azaleas if they are pruned late summer/fall or early spring will not bloom that year because you are cutting off flower buds that have formed. I think it is too late for this year already.

Depending on the kind of bush, some varieties don't respond to heavy shearing well and should not be pruned into the "dead zone" beyond the greenery.