As far as I know - and I'm no expert by any means - Acers should NOT be pruned at all. Just a gentle removal of any dead branches.

thanks for that, this a young red acer in a pot and has become very spindly this year. There are about 6 very long stems which we want to shorten and would like to know the best time to do it.

When the plant is dormant, between late autumn and mid winter is best. Make sure tools are sharp and clean to avoid possibility of disease and die-back.

Thanks will give that a try.