I very occasionally end up watching cricket, because there is something hypnotic about watching each ball and seeing what will happen this time. Also the strategy can get very exciting. But something like a football match or tennis is bewildering to me. I get playing sport, but watching other people play is weird.

It may make more sense to obsess over one, like Supernatural, that has no end in sight. ;-)
Ha ha. Well it is certainly convenient that I got hooked on what turned into the longest running American sci-fi show ever. :D

I'm alarmingly fickle. It's rare that I will watch any television show past 5-7 seasons.
I think part of the fandom gene is that we obsess like crazy about something for a few years and then move on. Buffy consumed my life for a few years, now I look back and wonder why.

If it weren't for the internet fandom, I'd have stopped obsessing over Buffy about five years prior to when I actually did.
Hard for me to say what would have happened. Prior to Buffy I didn't know such a thing as fandom or fanfic existed, but then I only looked on the internet because I was obsessed in the first place.

I'm usually done at two seasons, because the characters aren't evolving enough for me
Is this affected by if you are watching each episode as it is dripped out versus if you are binge watching? I would imagine many shows are fine if viewed once a week that would be intolerable in a binge.

I also require a strong female character be in there somewhere at some point.
Fair enough. I probably wouldn't watch if there was no strong male character, but luckily no show in the history of television has ever lacked a strong male character so I'm not restricted.

Supernatural finally lost me, because the female characters dropped by the wayside
I admit I was very sad when they killed Charlie. Most of the other females were just passing minor chars, however much they meant to the boys in-show, but Charlie was brilliant and it felt such a waste to kill her. I keep hoping they will find a way to bring her back. But I am an incorrigible optimist who keeps hoping they will find a way to bring John Winchester back.

it's rare for me to stick past six or seven seasons, unless the show evolves and people/characters change a great deal -- which only happens with serialized television shows, like Grey's Anatomy or a soap opera.
I've only watched a handful of shows that went on longer than that, simply because most of them get cancelled. British shows are never that long lasting, Doctor Who being the only exception. Of the American shows I think only Stargate and Bones lasted more than 6 seasons. And they both definitely deteriorated by the end, but I stuck with them because I like to complete sets of things so I would have felt it wrong to just give up.