When do I prune dwarf weigla bushes

In my humble opinion, I would leave them for now. Severe pruning does set some plants back. Pruning is best done after flowering. I also only lightly prune mine after flowering, as it has reached the height I want it at. hth!

My rule of thumb is:
Any shrub (or climber) that flowers in the first half of the year, ie before the enid of June, should be pruned immediately after pruning
If it flowers in the second half of the year, prune it in early spring, just as growth starts

Early last year I wrote a couple of blogs on pruning - you may find them helpful

I prune my after it has flowered but only to keep it within its alloted space.

We have never bothered pruning any of ours since I want them to get BIG! But agree, wait till they have flowered if you need to keep them small.

Hi Cdf and welcome to GoY!
Wiegela flowers on the previous years' growth so a haircut immediately after the end of flowering is the certainly the right way to do it, then they have the whole summer to put on new growth. I've found you can cut them back very hard and they seem to come back for the next year without any difficulty.