well, just about forever. i just got a new computer yesterday, and in it was a packed of silica gel. this reminded me of a show i watched a few years back about a lady that offered flower arrangements that lasted months if not longer. i think her business was in n.y., and i wish i could remember the name of it to share her work here. she showed the process she used to preserve beautiful fresh flowers that would last a very long time. i was so impressed with the way they looked. it was like they were frozen in time right at the peak of their beauty. to preserved them she used silica gel. she had shelves full of tupperware containers full of silica gel and the loveliest flowers. the method is simple:

1. pick flowers at their peak, and cut off the stem, leaving about an inch
2. fill an air tight container with and inch or two of silica gel
3. gently lay the flowers, face up, inside, spaced apart so they don't touch each other
4. gently and slowly pour more silica gel over the flowers, covering them completely, trying not to crush them
5. seal the container and set aside for 48 hours to a week
6. check the flowers to see if they are dry. if not, recover and let them sit longer
7. once they are dry you can place a wire stem wrapped in floral tape around them and arrange them

flowers that work well: roses, carnations, hydrangeas, gardenias, dendrobium, calla lilies
flowers that are harder to preserve: tulips, irises, daffodils

you can find silica gel here.

more detailed instructions can be found here

I found a great website that offers arrangements that are preserved in this way, Guirnaldas.
check out what they offer, they are gorgeous: