I need to make gumpaste flowers for a wedding cake. How far in advance can I make the flowers?

when ever they will last as long as you keep them dry and dust free.

Any time you want get started
As the other poster said, they will keep forever as long as they are kept dry, dust free and out of light.

You let them dry for 24-48 hours then they can be stored. The don't need to be in a sealed container - a cardboard box is sufficient.

The flowers will keep really well after drying in a plastic container if you place a silica gel packet in with the flowers. Once the gel inside turns blue, then it is no longer absorbing moisture. These are the little packets that are found in new clothes and shoes. I usually put a piece of paper towel between the packet and the flowers.

Michele