Normally the flowers only last a few days, but they should flower all year round given good light and conditions.

I have personally found the flowers to open (or 'ripen' in the case of grand duke) around dawn and by night they have lost their prime. The flowers will have started to decline and are already ready to fall, just waiting for some wind to knock them off but can stick on for a few days longer. But the good thing is, a nice healthy plants will continually produce flushes of flowers in the growing season giving constant pleasure of fragrance.
Some sambac varieties last a little bit longer than others.

It's true, Jasmine sambac flowers last a few days, but if given sufficient light, the good news is they bloom year round, especially Maid of Orleans..Even in mid-winter when most other plants are asleep.

Re: your cestrum,

lots of sun, lots of water, and once it reaches a mature size (how big is it now?) it should start popping out blooms! A little fertilizer can't hurt, either. Just make sure it isn't drying out too much in between waterings, with this weather you may need to water daily, and put a saucer underneath the pot so it's got a little "reservoir" during the day -- these are thirsty plants!

Hope this helps!

Jim