How small should you compress a down bag (850+ fill WM bag)? How long is it safe to keep it compressed?

Are there some guidelines somewhere or is it pure superstition and old wives tales? I read nothing about it on the Feathered Friends or Western Mountaineering websites.

I plan on keeping mine either hanging up in the closest or in the storage bag, but I wasn't sure what the guidelines were when actually on a hike or in luggage enroute.

Thank ya'll again,

Nitrosteel