title_body: Are there any rules of thumb for number of "steps" between two points in a language's natural development? Let's suppose that you want to create Conlang B. You want this conlang to be derived from Conlang A. Let's also suppose that you have some arbitrary period of time in between A and B. Are there any rules of thumb for how many steps&mdash;how many major grammar/vocabulary alterations, etc.&mdash;would generally manifest between A and B as a general guideline? Is it entirely up to the conlanger's prerogative, or can patterns from natural languages be found?
upvoted_answer: It’s relatively hard to say. First, some general numbers:
  
  Over 1000 years, Old High German split into the various southern German dialects, including Swiss German and Standard German. While differences are very noticable, many developments were parallel and speakers usually don’t take long to get accustomed to the differences. A similar time depth can be claimed for English vs Scots. Old High German itself is not readily intelligible to the modern speaker, but many words are recognizable.
  
  2000 years ago, we had Latin and Proto-Germanic, the ancestors of the Romance and Germanic branches of Indo-European. While the modern languages all show clear similarities in most regards of the language, they are as a general rule not mutually intelligible and speakers need to invest a lot of time to learn another of those languages - but still much less than with an unrelated language. Proto-Germanic and Latin are both quite different than the modern languages, showing vast differences in morphology and syntax, as well as having undergone many sound changes since. Nevertheless, with some knowledge on etymology, words can readily be recognized for what they are in a written text. 
  
  About 4000-6000 years ago, we have PIE. At this point, the language is no longer immediately recognizable as being related to a modern European language. The grammar is very different - just as an example, PIE verbs inflect for three persons and three numbers, but not directly for tense or aspect, which are instead marked with various more-or-less productive derivational affixes. The phonology would also look very out of place in modern day Europe. And for most roots, even if they didn’t change much phonologically, there was probably a semantic shift somewhere between PIE and English. The average English speaker will understand exactly zero words in a passage in Serbian or Punjabi.
  
  
  
  But there’s another side to this as well. General tendencies are fine, but what about individual languages? If you compare Italian and French to Latin, then it’s plainly obvious that French has changed more, at least with regards to phonology and morphology. Similarly, comparing English to Icelandic, the latter is clearly more conservative (though no where near as much as some people might make you believe). To me, French and English look like they are 500+ years further in the future than their contemporary siblings — in reality of course they just underwent some more drastical sound changes that in turn messed with their morphology, which in turn forced changes to the syntax. People try to ascribe those changes to various factors, usually external ones such as a large number of foreign learners, but most of this is not grounded in actual science and just speculation.
  
  As such, no definitive answer can be given. I find the time depths I gave above to be good rules of thumb, but single languages may deviate from those quite a bit. To achieve such results, it’s important to apply changes not only to the phonology, but to all parts of the language — especially the lexicon.