What I need to say here is that all I can do is to quote the law and to try to explain it.
I cannot change what the law says.
I fully admit that it can sound a bit strange when one tries to apply the law to certain scenarios, imaginary or even sometimes real.
Personally, I would like to see some kind of time limit in the canons. I don’t know what that might be. Maybe 100 years? Maybe 4 generations? Maybe 200 years or 10 generations? I cannot imagine anyone expecting people to go back 1000 years into their family histories. But the simple fact is that I cannot change anything. Posting the question “how long…?” here on CAF won’t change anything–no matter who responds, the answer is still going to be “the law says that there is no time limit.”
I can say from experience, that the Catholic Church does not demand or expect that people research their ancestry centuries into the past. In practice, here in the U.S., we typically go as far back as the country of emigration, and we do not try to go back any further unless there is something a-typical about what we find. Nevertheless, if a person could show records, no matter how far back they go, that person would have a canonical claim to membership in a specific Church based on the canons (a very canonically solid one).
I also want to repeat what I’ve been saying here about the recent changes made by HH Francis because they’re very relevant to the discussion. If someone is in a position of having a desire to change from one ritual Church to another, that is now rather easy to do. Remember that the canon that says membership goes back according to the paternal line with no time limit is not to be viewed in a vacuum. Those canons (both codes) do not mean that it is impossible to change Churches. That is very much possible. Changing membership from one Church sui iuris to another is not a matter of attending or registering (which is meaningless in canon law). One may change Churches, but this must be done according to the canonical process.
Just please understand that addressing the question to me (or for that matter, anyone on CAF) of “how far does it go back?” is an exercise in futility. The answer is “there is no time limit” and that answer won’t change no matter how often the question is posted.
Also, I do not recall exactly what changes HH Francis recently made. I know he made things easier, but I don’t remember exactly what he changed, and frankly, I run the risk of confusing this with the Anglican changes because I’m dealing with that pastorally right now. I might post something too quickly because I’m thinking about something else while I’m at my desk multitasking. The point here is that I am directing people to look at the changes he made for changing membership from one Church sui iuris to another.