All of the sentences are natural and correct. You could also say "Flowers wilt quickly" meaning they droop after being cut.
I really like the first sentence "flowers don't last long" - in grammar this is called 'negation'. In British English we often speak using negation, to say what something is not, rather than what it is, for example: "It isn't so sunny today" or "It's not too cold". I hope it helps :)