The longevity of cut flowers in a vase depends on how you care for them.

Bacteria kill flowers faster than anything else.  For your arrangements always use clean containers and change the water regularly to help the flowers last longer.  Fill the vase with water at room temperature.  Add the floral preservative that is provided with the bouquet; this will prevent the bacteria from clogging the stems and deliver nutrition to the flower heads, boosting their freshness and vibrancy in the arrangement.

Before giving the stems a fresh cut (if bought at the market) remove their damaged foliage as well as those that will end up below the waterline in the container.  Foliage in water rots and encourages bacteria growth and stem blockage

Next remove damaged and bruised petals where needed on the flower heads. For example, remove the outer few petals of roses if they look damaged or bruised. This will give them a fresh healthy look, help them open up quicker, and last longer.

There are three main types of stems: Green stems, Woody stems, and Hollow stems.

Green stems are slender, green, and bend easily and should be cut in water in a sink or a bowl, with a sharp knife or scissors at a 45 degree angle to prevent air getting into them. Otherwise a blockage occurs and the stem water uptake becomes inhibited.  Usually one or two inches are cut off; however this amount varies according to the flower variety and the stem length.

Woody stems are stiff and resistant.  Follow the above steps, however, after you make the new cut, take the stalks out of the water one by one and smash the ends with a hammer or cut a slit into the sides with a knife. This slit should be no longer than one third of the stem length to enable a free flow of water and nutrients up the stalk.

Hollow stem flowers such as Shasta Daisies, Dahlias, Poppies, Poinsettias and so on require more attention.  When you give these stalks a fresh cut, a white substance leaks out at the spot. This white fluid is a nutrient that the flower needs or it will die.  You will have to seal the stem ends with flame (match or candle) immediately.  This will help the flower to contain the nutrient inside.  After that, place the flowers in water at room temperature.

For daily maintenance, make sure to check the preservative solution level in the container.  Replenish as needed.  Remove any damaged or dead flower or foliage.  Cut an inch off the stalks every 4 to 5 days to maintain their water upkeep.

Enjoy grooming and caring for your flowers and keep in mind that they love it and definitely appreciate it!