I had a table at one place I lived and I actually called it the "sweet spot" for cut flowers. It got dappled sunlight most of the day and plenty of fresh air because it was near a window that I opened frequently. Anyway, I kept roses for up to 3 weeks and cosmos, shasta daisies or zinnias for up to four weeks in a vase there, looking very nice. I used the method I explained above, a bit of sugar, and nip the stems diagonally about every three days or when the water started getting a little cloudy.