I try to give most things that I was at least somewhat interested in originally at least a full episode. There's honestly been quite a number of shows I dropped numerous seasons in, 5, 6, even 10 or 12. Sometimes it's due to a cast change but often it's just my level of interest had been lowering and lowering for a while and there was some sort of break (either mid-season or between seasons) and I got out of the habit of tuning in to X channel at Y time (for shows airing on regular tv) or I just never noticed or cared enough to watch when a new season dropped on a streaming service.

I'm not sure how what the longest I watched something before quitting I think I lasted until after s10 of, ER, NCIS, and Supernatural so over 200 episodes. And then there's soap operas, many of which I watched for years and years before wandering away and they air 250 episodes (ish) a year so probably well over 1k episodes if not much, much more.