My goal for 2013 is to really get a 6 month supply. Ideally a year would be great, but I'm starting small. I would say start with a 3 month, but I think if we really needed to we could get by for 3 months. Well minus water!
I came across a blog called Prepared LDS Family. It it a very nice reference, one of those this is what you should buy this month type of things. I'm probably going to be referencing it a lot next year. Here is the link.