How long can orchids be safely submerged in water?

You should put them in water right away, cut the stem on an angle so they can drink the water and not get clogged leave the flowers on the stem until you are ready to make the arrangements, also if they come with flower food use it and like the other person said put them in the refrigerator, but in the front, not the back, it's colder in the back, that will cause them to wilt, I would make the arrangement the night before the wedding, they can last up to 3 weeks in water. Good luck and congratulations to you.