I watched about 14-15 seasons of OG Law and Order before realizing that it had turned into mostly stale rehashed stories and big name celebrities guest starring for ratings.

If a show I've been watching for a few seasons has 3 bad episodes in a row I usually stop watching for a while to decide if I want to continue or not.

If it's a show I just started watching if 2 of the first 5 episodes are bad I'm out. If they're not actually bad but just soso I'll usually decide at the end of the first season.