When do you prune box

I assume you are not talking about boxwood rejuvination just maintaining size /shape?

Box wood does not put on a growth spurt in spring so all you need to do is a light clipping to maintain size/shape you can do it any time in the growing season.
I do mind in late spring and again around july/august. I lay an old sheet around to collect the trimmings.
I see that square ones have come back into fashion - mine are all round ah well I like them!

Also, make sure you dispose of the trimmings safely....as they dry out they become toxic...

Oh didn't know that Aa.....like you Wessan I have a small box planted in a terracotta tub, I trimmed it a few weeks ago and it's fine...I'm hoping to get it the ball shape like Drc726's.

I know squares the new round! but I like mine and they are very easy with those new hand held pruning scissors.