How long is it safe to hold a wedding dress in a garment bag?

If the bag is fabric, it should be okay. You’ll want to get it pressed shortly before the wedding.

If it’s plastic, don’t. Get a cloth bag or wrap it in a sheet; the fabric needs to breathe. Don’t put it in a garage, attic, or other non-climate controlled space, keep it in a regular house closet, but don’t squish it- give it space. If you use a block of cedar to keep bugs away, like another poster recommended, make sure that it doesn’t touch the dress- the oils in cedar will damage the fabric. A museum curator told my mother this- that she needed to line her cedar chest with acid-free board if she didn’t want her family heirloom clothes and quilts to deteriorate.