Buddleiahs are best pruned in the early spring, after winter frosts, if you mean the commonest species which is davidii. If you prune in the autumn and they make new growth it will get frosted. Pruned in the spring, just before they start into new growth, they will make a lot of new growth on which the flowers will be borne. Cut them right down as low to the ground as you want, as each cut shoot will produce more buds.
One exception is buddleiah globosa, which has round orange flowers, as this has to be treated more like a normal tree and pruned sparingly, as the flowers are borne on older wood.

I usually prune hard in March, but last year I was very late in fact it was May and all that happened was it flowered later:-)

if its a very large bush you could reduce it by a 1/3 to reduce wind rock/ snow damage. then in spring you can be quite brtal, cutting down to about 2-3ft high.