When to prune a twisted acacia tree

Question

We have a very tall but spindly twisted acacia tree must be about 20 years old now and when is the best time to prune and how hard can we do it?

Answer 1

With what you describe, tall and splindly, it may be wise to prune as soon as possible in order to avoid potential wind damage -breakage in winter. If the tree is staked or otherwise protected from high winds and snow/ice loads, there is some advantage to waiting until spring when the new growth will come soon after pruning.
Don't cut too hard. Limit the crown reduction pruning to no more than 25% of the total foliage to be removed in a one or two year period of time. Otherwise there is too much physiological stress of leaf loss for photosynthesis of sugars to nourish the plant, and heavy pruning creates an adverse heavy growth response that is ugly and creates more maintenance.
https://www.gardeningknowhow.com/ornamental/trees/acacia/trimming-acacia-trees.htm