The moisture in the vanilla beans will cause the sugar to clump over time. I always recommend shaking the jar every 1-2 weeks, but after about a month of curing, you can actually dehydrate the sugar crystals. Line a sheet pan with parchment paper and spread the sugar in an even layer. Let it bake at the lowest temperature in your oven for about an hour. It'll clump up, but that's good! Let it cool on the counter, then crumble the sugar. It'll flow freely (unless you live somewhere extremely humid). I use this tip for making fresh lavender sugar, and it works wonderfully every time!