The bleaching effect is from exposure to ultraviolet, so YMMV depending on the amount of UV. If you live in Seattle, it will take longer than in Arizona, for example, because the sun shines less frequently in the soggy northwest, and there is more atmospheric stuff (rain, fog, snow, etc.) to get in the way. Just check them periodically.
Yes, they will slowly yellow again, and UV is friendly to nothing human made. After such exposure, recoating with decal film might not be a bad idea.