Well, not exactly forever, but much longer. We all know that stems should be cut on an angle with a sharp knife, strip all leaves below the water line, change the water periodically and maybe add a packet of powder we get from the florist.
But here’s the thing. After a couple of days, the water gets all cloudy. Supposedly, that is caused by bacterial growth and the bacteria colonize the cut ends and plug the tubules so the flowers can’t aspirate water. So I have been told.
Where does the bacteria come from? Bacteria are everywhere including on the flower stems just waiting for water and nutrients. So make a mild bleach solution in the vase higher than the planned water level, insert the cut flowers and let them sit for a couple of minutes, decant the bleach, add fresh water and the flowers and you are in business.
Last winter, I had carnations which lasted over two and almost three weeks with a couple of changes of water. Also, recut the ends when you change water.
I worked at florist shop awhile, cool air is one of the best thing you can do for cut flowers. Heated rooms are the fastest way to make them wilt. Florists keep all their cuts in a refrigerated room or cooler in containers with very little water.
For the same reason we keep lettuce in the refrigerator. The problem is that a floral bouquet doesn’t fit too well in a refrigerator unless it is set up for that purpose.
Also, I wonder if flowers would thrive in the bleach solution, or if they would quickly succumb or maybe smell bad. Maybe an experiment is in order. Divide the bouquet in two, one with bleach solution and one with water only and see what happens.
Along with everything else mentioned, my gf (who does flower arranging as a hobby) uses green floral wire to support the stem where it’s likely to bend. You cannot see the wire, and by keeping the flower from bending it keeps the channels (phloem/xylem?) open.