It depends on both the weather and the flower. They're all different.

As Brittie said, the weather (rain or heat) can affect the life of a bloom, and also some bushes tend to hold the blooms longer. For me, the blooms on Fame just hang on and on, even when they fade they stay on the bush and look nice, at least from a distance!

Let me rephrase that. Under the best conditions, what is the longest a rose CAN last.

As the previous posters stated, it is both the weather & genetics of the rose that dictates flower life span. If you are looking for roses with long garden & vase life, I've found that "florist" HT's usually fit the bill the best. Some that I have are Tineke & several Kordes Freelander roses, Antique Caramel Freelander being one of my favs. Also another long lasting bloom is Veteran's Honor. Many of these blooms will last for 10-14 days & sometimes more provided you keep them well watered, especially during hot spells. Many of the Freelander roses can be purchased thru Palatine & Pickering.

I've found making flowers last a bit longer is a bit of skill and a bit of art. It took me awhile to learn what works and what doesn't (or what I'm not good at maintaining). There are some basic tips you can follow; Some of the neat ideas include adding a little bit of Soda to your flowers. Rather than go into them all here, I recommend just checking out Alma Flowers post on the 5 DIY Tricks for helping your flowers last longer. Hope this helps.