+1 on making sure they're sealed and putting them in the freezer. I brewed an all-grain that turned out pretty well even though I couldn't get to it for 2-3 months. Once I realized I wasn't going to be able to brew it right away (as in a few days after getting my grain, which was already crushed), I wrapped them in additional plastic wrap and put them in the freezer. I still wouldn't do it if I can avoid it, but my beer didn't seem to suffer too much for it. I would think that you would have even less to worry in this case since it is an extract kit. So based on my experience, I'd say you can get away with up to three months if you need to. I can't speak with experience on any more than that.