Good quality roses cost a great deal of money, so they should last for quite some time in a vase full of water. With the right kind of care and preparation, they can last for 2 and up to 3 weeks. You can do several things to keep cut roses going strong, including adding substances to the water that will help them look fresh longer.
Cut the ends of the stem. When roses are received they are sometimes in individual water tubes. Remove them, because the water will soak up fast. Fill a clean bucket with room-temperature water. Place the stems of the roses in the bucket and while they are still under water, cut the stems with a sharp knife about 1 1/2 to 2 inches from the bottom at an angle. The stems can also be cut under running water in the sink, but it is not quite as successful. The cut is made under water so that air bubbles will not go up the stem and prevent water from being sucked up into the stem to the flower. The cut is at an angle so that when the stems hit the bottom of the vase there is still an area away from the vase that can conduct water up the stem.
Remove any leaves that might be below the water line in the vase. Leaves carry harmful bacteria and chemicals which will make the roses deteriorate much faster. Pick them off carefully, making sure not to damage the green bark of the stem. If that happens, a bubble can enter the stem and prevent water intake to the flower.
Add the floral preservative that came with the roses. Preservative can be purchased separately from flowers at a florist or garden center. Follow the directions as to how much to mix with water and add the preservative to room-temperature water. The preservative adds sugar to nourish the flower, bleach to keep away harmful bacteria and acid that balances the pH level of the tap water. All of this improves the longevity of the blooms.
Add water and more preservative when needed. After just one day, the roses will suck up about half the water in the vase. Days after will not be as extreme. Add just a little water after the first day, then add water with preservative when needed. If the water gets cloudy, replace all water immediately.