Q:

How much time does it take to create/fork a new language? 

I wonder if there exists any summary or paper analysing the time it takes for the creation of a new language (with all reservations concerning definitions of languages et dialects etc.)? 
Take, for example, the Romance languages, which are derived from Vulgar Latin.  It would be (I suppose) around 400 years (between 4th and 8th century), maybe a little less, and for Yiddish some 500 years (between 10th and 15th century). 

A:

The question you're asking is probably not answerable, but if we just focus on one area where a lot of work has been done, maybe we can get closer to answering it. 
Glottochronology as originally conceived by Morris Swadesh is one approach that tries to answer this question in a specific area.  It's very controversial, but a lot of research in this area is built on variants of Swadesh's idea, sometimes in an evolutionary biological or computational context. 
The underlying assumption in glottochronology is that there is a core vocabulary for any given language that is relatively stable and changes at a constant rate analogous to radioactive decay or gene replacement.  Numbers are obtained by considering data from languages whose chronological dates and degrees of separation are fairly well-known (e.g. Indo-European).  By mapping the rate of change or retention, language divergence can be measured.  The accuracy and usefulness of the results, on the other hand, is much debated. 
Some of these earlier papers might be of interest:
Bergland, K., & Vogt, H. (1962).  On the validity of glottochronology
Chrétien, C. D. (1962).  The mathematical models of glottochronology
Dyen, I., James, A. T., & Cole, J. W. L. (1967).  Language divergence and estimated word retention rate
More recent works:
Gray, R. D., & Atkinson, Q. D. (2003).  Language-tree divergence times support the Anatolian theory of Indo-European origin
Heggarty, P., Maguire, W., & McMahon, A. (2010).  Splits or waves?  Trees or webs?  How divergence measures and network analysis can unravel language histories
Renfrew, C., McMahon, A., & Trask, L. (Eds.) ( 2000).  Time depth in historical linguistics

