It would depend what you wanted to prune it for. Usually, it's inadvisable to prune Magnolias unless it's to keep them in check and stop them getting too big or grow over pathways etc.

However, if you prune it hard, you might not get any flowers for a few years.

As Andy says, it is generally inadvisable to prune magnolias.
If it has to be done, do it when the threat of frost has passed - I'd say mid to late May, according to where you live in the country. That way, it has the rest of the growing season to recover.