Of course, I don't mean when is it time to quit genealogy, for me, that will never happen until I die; I mean when is it time to quit looking for that elusive ancestor? Oh, guess what? The answer is the same. We never stop. But there are limitations to our searching and fixating on one ancestor or even one line can sometimes be very unproductive. Genealogy is to some extent an art and spending too much time on one topic can be stagnating and lead to burnout.
The Genealogical Proof Standards (GPS) uses the term "reasonable" in conjunction with the term "exhaustive" as in a "reasonably exhaustive search." Of course that raises at least two more questions; what is reasonable and what is exhaustive? Let's tackle the exhaustive part first.
What is an exhaustive search? Is it one that exhausts you or one that exhausts the reasonably available sources? The GPS explains this by saying "Assumes examination of a wide range of high quality sources" and "Minimizes the probability that undiscovered evidence will overturn a too-hasty conclusion." Guess what? More questions. What is a "wide range" and what are "high quality sources?" Is there some source certification bureau that certifies sources as high quality? What does it mean to do a "wide range" search?
Maybe you can see my point. How much evidence you obtain and whether or not that evidence is convincing is largely subjective. As a genealogical researcher, you have to make the effort to find enough evidence to convince yourself that what you have found is sufficient to constitute a reasonably exhaustive search. For example, looking in an online database such as Ancestry.com and calling it complete and reasonable would probably not convince anyone. But spending the time to adequately document your search and looking in a variety of different repositories for a number of different types of records might be both reasonable and exhaustive.
Let me give an examples:
I know my seventh great-grandfather was born and died in Rhode Island. I check Ancestry.com, WorldVitalRecords.com, FamilySearch.org and do a Google search. Is that enough to be reasonably exhaustive?
Well, the answer you might give might be yes, but I would definitely say no. None of these searches are so complete and cover the geographic area some exhaustively to be sufficient. If you take the time to learn a little bit about research in New England, you know exactly why I would have that opinion.
What if I added the New England Historical and Genealogical Society records? Well, you are getting warmer but still not there. I would not be comfortable that the records had been searched until you checked local repositories in Rhode Island. Until the local records were consulted, all conclusions drawn from the previous research would be tentative and open to changes from further undiscovered evidence. I would also assume your search would include local newspapers of the time period and cemetery records.
But what if you do your reasonably exhaustive search and do not turn up any records at all? What if you are drawing a blank? Well, unfortunately, negative evidence might be helpful and it might not, depending on the circumstances. You may be looking in the wrong place (very common) or looking for the wrong person (not as common but still possible) or even looking for the wrong family altogether (not as uncommon as you might think). If you cannot come up with even one shred of evidence connecting the dots back in time, you need to give up and move forward in time to the next generation back from your ancestor. You may have to do this two or three generations until you start to find adequate sources that give you a firmer foundation for your research into the past. You cannot build research on a shaky foundation. You need to have as much information as is possible about each generation so that there is no question you are looking for the right person, at the right time and in the right place.
Is there a practical point at which you can justifiably quit? Yes, when you have really reached the examination of a wide range of high quality sources and cannot find any more sources. A wide range of sources usually refers to different types of complimentary sources, such as, probate records, newspaper accounts, land and property records, census records (if they exist), vital records, church records, school records, directories etc.
If you can truthfully say that you have looked everywhere suggested by a long list of record types, then you can reasonably consider your search to be over (until the next time you think about it).