I've never been particularly concerned with the length of a tv series when considering whether to start watching it. I'm perfectly fine with dropping it halfway through the series if it manages to lose my interest for more than a season.
I also tend to pick up tv seasons on DVD rather than streaming, so I only ever watch one or two seasons at a time as I happen to find them for sale inexpensively at the usual used places. If the first season looks interesting, I'll get it, and if I like it, I'll get the second, and so on.
I've got all 8 seasons of GoT, all five of Leverage (I watch the whole series at least once a year), up to season 10 of NCIS: LA, the first seven of Arrow, and the first five (six?) of Flash. I got to about season six of Dexter before I started losing interest.
I also used to catch a lot of stuff on tv as reruns (they used to rerun whole seasons on TNT in the afternoons) before I cut the cord and cancelled my tv service.
I've seen every episode of Star Trek: TNG, every episode of the original Law & Order, and about the first six or seven seasons of SVU... Unfortunately, TNT stopped running SVU in reruns after that, and my work schedule didn't allow me to catch the later current seasons as they aired.
So, I guess I'd say the longest tv show I've ever "committed" to watching after its original airing is probably Law & Order...