Cheers for the shoutout /u/jelvinjs7, thoughi 'd suggest a slight rewording of your first sentence, unless I'm misunderstanding the intended meaning.
It's not that languages change at different rates but those rates are internally consistent per language. It's that language, in general, changes at different rates, even for a single language through its restricted history. So it's not like Chinese changes at one rate and English at another, but that Chinese throughout a period of time doesn't change at a steady rate nor do all aspects of the language (phonology, lexicon, syntax) change at the same rate at the same time either.
So the real issue u/tjmaxal is that we can answer your question for a given language based on historical records and historical reconstructions, but we can't just apply a single formula to a given language and get consistent answers.
To really answer, we'd need to know what your native language is, and we'd need to have some sense of what the history of that language has been for a period of time within which we'd send you back. This is fine with languages with well documented histories like English or Burmese, but not as easy (or even possible at all) if your native language is Pirahã.
Basically, the better documented the language is in historical records, and the more closely related language varieties there are alive today, the better we can answer the question, but that answer will be completely unique to that language.
If you feel like sharing your native language and assuming it's not English or Mandarin (addressed by /u/jelvinjs7 already), we can take a stab at the answer.
Well that depends partially on what your native language is, since languages change at different rates. The FAQ has a whole section about language’s role and application in history, including bits and pieces about how languages changed and developed. I can only find two languages for the “how far back could I still communicate” type of question, though: English and Mandarin.