I'm not sure how your zone 9 will affect pruning recommendations. Here in zone 6a, where we get reasonably cold winters, those are typically pruned at the end of winter or beginning of spring before they come out of winter dormancy. And then they are left alone the rest of the season so as not to interfere with the late summer bloom.

If it was just planted, you may want to go easy on the pruning while the shrub gets established though.. hopefully someone else can chime in about that if it's an issue.

It is usually not necessary to prune hydrangeas, unless you mean dead wood/flwers (which can be pruned at any time), get a lot of snow or unless it is so big that it does not fit. I leave mine alone and prune dead wood in May. Limelight gets quite large. It can attain 10+ feet high/wide in some locations.

Why are you planning to prune your Limelight? As Luis said, it isn’t needed for the plants’s sake other than removal of dead wood or rubbing branches. I do deadhead my old blooms in early spring before new growth starts (and some folks remove the old dried blooms as soon as they brown) but I find that in general this type of plant has a pleasing shape and doesn’t need any real pruning.

IME, trying to keep a large panicled Hydrangea such as Limelight at a given size by pruning is an exercise in futility since they regrow quickly each year. So if you are pruning for size, move your Limelight to somewhere it can grow to full size and plant a smaller panicled Hydrangea such as Little Lime or Bobo in its place.

If you are not pruning for size, let it grow for at least three years before pruning so it has time to grow roots and get established rather than using resources to replace the branches and leaves you remove in pruning.

If you plant it in a spot that it outgrows, pruning may be necessary and often you will have great results. Here is Limelight I got my dad a few years ago (while he was still living). It quickly outgrew its spot so I pruned it fairly hard and now it must be pruned every year to keep it in bounds.

August 7, 2015 and it has consumed a lot of area and leaving little room next to the house.

A good illustration of why to pick a plant that fits the spot