There’s no single “right” time to trim, mostly, it’s whenever your hedge needs it. That said, autumn is a great option, since your hedge stops growing over winter and you get the longest possible stretch of it looking sharp, and trimming just after the big spring growth spurt is another good window, since growth naturally slows through summer afterwards.
Once we’re on-site, we’ll trim back to the existing line, set a new (often lower) line if you’re after more light, or simply straighten things up so the hedge is easy for you to maintain yourself going forward – formal hedges, privacy screens or decorative shapes, we’ll talk it through with you before we start.