March April is usually a decent time to do it. I'd cut it down to about 3ft this year and then next year remove a bit more if it is still too big. It flowers on new wood. so if you didn't prune it you would still get flowers.

I trimmed mine back approx halfway two weeks ago,mainly so the branches didn't get snapped off by strong winds..and then I shall prune it further as suggested above.you should be able to see new growth lower down a branch,so use that as your pruning guide...I've done this most years,and it always flowers well..

It’s very hard to kill a Buddleia. You can trim it now if you want though I wouldn’t take too much off. You can always prune it a bit more in March/April.