I'm looking at making some isomalt gems and was wondering how far in advance I can make them. How long to they keep?

Isomalt decorations can pretty much last forever as long as they're kept in a cool, dry environment. Once you make the gems, put them into a covered airtight container along with some food-grade desiccant packages and your good to go.

Best way to store them is in the covered airtight container with the desiccant and then place the container into a cardboard box away from humidity and strong UV lighting (sunlight). They can stay that way for weeks, months or years with no ill effects.

I've seen isomalt showpiece decorations that have sat out on a shelf for over a year and they still look the same as the day they were made.