“Stop watching before it gets bad” cut off point for series that you like?

I think its axiom of life that most long running shows (and some not so long running…) WILL eventually get bad – the Seasonal Rot trope is a manifestation of the entropy of the universe.

So, in series that you overall like but eventually saw a dip in quality or had a bad ending that retroactively ruined it (… you know what I’m talking about) what is the point in which you would say “hey check this out, but just know that it gets bad after X”

My go to example is the Vikings show, it just ended earlier this year so I won’t post too many spoilers. Its pretty universally agreed by fans that out of the 6 seasons (actually more like 9, because seasons 4, 5 and 6 where twice as long as 1, 2 and 3 and where broadcasted in separate times of the year) 1 to 3 where the best ones, 4 was when it started going downhill, and 5 and 6 were bad because the series lost focus on what it wanted to be. IMO, my cut off point was actually the end of SE2 because, for some reasons SE2 actually ended as if it were a series finale with emotional closure and tight up lose ends; all the others end in cliff hangers, so it does literally work as a literal cut off point