Couple of questions re k-1 visa process

Hi, I just found your blog today and it's great, so informative thanks! Anyway I very recently met a lovely American guy on-line and we've already arranged to meet in person in Florida where he lives as we're both very serious about wanting to settle down. We're speaking regularly on the phone and emailing and so far so good!! My question is whether you have to be in a relationship together for a minimum of 2 years before you can apply for a K-1 visa or whether at any time during the first 2 years you can do this as it's a little confusing? Also which vaccinations are required for the medical

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