How Far Back Do You Go?

I typically imagine a language in a window of about, I don't know, 50 to 300 years depending on what I think is going on. So I do very mild historical stuff just because that gives me things like vocabulary and grammar which look somewhat naturalistic. But the really deep create a language, derive another language from it through a thousand years. I've found that when you have an inflectional system in place in your proto language and then you derive another language using sound changes, everything goes bug nuts.