If you want to add daffs to other flower in a bouquet, cut the stems under water and let stand in water for 24 hours. By then the chemical that kills other flowers will be gone ( bled out) and you can add the daffodils to the flowers in another vase. Disgard the water you had them in.
i've had daffodils last about 2 weeks for me if i buy then when they are closed. we usually get them the end of February & beginning of March here at the grocery stores as cut flowers. so i buy a couple bunches to make it feel Spring like in late winter.
if you're cutting them try to get as close to the ground as possible & do what Larissa says when they are still closed. basically when the yellow/white/or other color is just starting to poke through the green of the flower covering is when you should cut them. the heads will still be upright at this stage. once the heads start drooping it seems they won't last as long in the vase (mason jars is what i use to keep mine in).