I'll give your question a go. First, in some cases, dialects and languages are for all intents and purposes the same. That does not mean in all cases, but in rare cases, they can be as different as either one is from another dialect or language. What distinguishes a language from a dialect in such cases is the cradle that embraces that language or dialect, made up of or lacking a military, a country, a poplulation, a history, a heritage; official recognition.
Sometimes, it's even more complicated depending on the theories of linguistics embraced or appealed to if to argue either or. For example, for some people, and indeed many, Chinese is a language. Point blank: it is its own language. It's not a dialect, it's not a family of languages, it is simply a langauge. Period. And in Chinese, there are several dialects, two of which are of course very well-known. Other people will argue, however, that no, this is wrong: that Cantonese is its own language, just as Mandarin is. And, from there, things start to get messy. I personally am of the view that Chinese is a language, and the latter, varieties or dialects therein. At any rate, for them to become a language, or for some language like Portuguese or Spanish or so forth to become languages, the factors I think are more relative to the period in which the languages came to be and what happened and in some cases has happened within and outside those languages (sciences, philosophies, etc) along the way.
In short, you might compare it to countries. When does a country become a country? Well, if you ask someone from Tawain if they have their own country, you can imagine that they will tell you that they do. They'll even give you all kinds of justifications: their own passport, their own government, military, etc. Embassies even. But if you look up the UN's list of countries, or that of the Department of Defense, you'll not see it listed as a country. So is it a country? Well, what makes a country a country is such that Tawain is about as close as you can get to a country without being officially recognized as such for political reasons. Languages are very much like this. There are all kinds of external factors that come into play when thinking of such in terms of language versus mere variety or dialect. It is not enough that internally the population should vie for such a status. For example, The Ivory Coast recently declared that it wishes to be called Côte D'ivoire, retaining its French identity. Or, for example, many people outside of Iran erroneously call Persian Farsi without knowing that Farsi is actually a transliteration of Arabic because Arabic doesn't have the P sound in its language, meaning that Farsi actually comes from Parsi, both of which are endonyms (or languag/place names) within the Persian language itself, the root of which is now used within the language to distinguish between the two dialects of Persian between Iran and Afghanistan but that nonetheless gave us the word in English that we already have to begin with: Persian. All too often people hear a country or place or language as pronounced or said in the original language and think that they have to say it in such a way if to say it correctly, but this is wrong. Farsi and Côte d'Ivoire are no exception. At any rate, as you can see, internal and external influences play a critical role in the determining of the status of a language. The rest of the world has to agree and for a long enough time, even if ultimately the status is merely a putative one.
So in short, even ignoring all the unstated factors for what makes a language an actual language versus something else, outside factors would seem to complicate the definition to so great a degree that trying to model language on a time line in the way that you ask could only best be done hypothetically such that through estimates from a couple hundred years, or in the longest cases, thousands, where given factors typically present in official languages could be observed and potentially later fade away or die out immediately, one could argue through induction that any given language could occur. In the minimal amount of time possible, like empires or rapid migrations or expansions etc, one might conclude that although considered a language locally or in surrounding regions, or even more broadly yet still hypothetically, only temporarily so, prior to dying or being overtaken, etc. In short, I don't think that 200 years is impossible depending on the initial sociolinguistic conditions, but I think that would likely be the absolute best-case scenario, where the only hurdle the protolanguage or dialect might be thought to have to overcome be something like state formation.