For a solid, rectangular or square style hedge, it's best to trim several times a year from Spring through to Autumn.
To keep the top level of your hedge, place a line of string just above the hedge as a guide for a straighter cut. A STIHL hedge trimmer will make the job easier for you and get a better, quicker result than with a pair of shears.
If your hedge is overgrown, get onto it in spring by pruning the side growth back almost all the way back to the main stem on one side only. Repeat on the other side the following spring (you might even be able to do it in late summer if the pruned side has regrown sufficiently).