When is the best time to cut back pysocarpus opulifolius 'diablo' please?

Early spring! It can be cut back quite harshly if you dont want it to grow to 10ft x 10ft it can and appears to have reached. It is suposed to tolerate poor soil conditions but prefers well drained, acidic soil. It is drought resistant and likes full sun.

I forgot to say 'Welcome to GOY',

Sorry Joanii, but have to disagree with you! Best time to prune is immediately after flowering, so end June approx. That way you won't cut off next year's flowers.

Is that in England or the US? I got my info from internet as I needed to check up as I am in two minds wether to plant mine or give them away.

Pruning after flowering towards end of June is what I do. I live in East Anglia.

The question was from USA and I researched several sites re: USA and gave me the info that I passed on.

If, like me, you're not too bothered about the flowers, then cutting it back almost to ground level in February or March works fine.
The result being a tidy plant, with long straight shoots which have the tips pinched out near your desired height.
Mine (in late June) is around 130 cm in height, and should reach around 180 cm or so by late Summer, even though we're very exposed to salt-laden sea winds, and situated 100 metres from the west cliff on Portland, Dorset (UK). It is one of the few deciduous shrubs that doesn't wind-burn during windy springs and summers here.
For that reason, I find it a very useful backdrop to smaller grasses, and bright, dwarf conifers, such as the vivid gold Chamaecyparis pisifera 'Filifera Aurea', and the startling blue Picea pungens 'Glauca Globosa'.
In summary, it is a very tough, very easy and very colourful shrub, that will be any shape or size (within reason) you want it to be.