It may make more sense to obsess over one, like Supernatural, that has no end in sight. ;-)
I'm alarmingly fickle. It's rare that I will watch any television show past 5-7 seasons.
If it weren't for the internet fandom, I'd have stopped obsessing over Buffy about five years prior to when I actually did.
I'm usually done at two seasons, because the characters aren't evolving enough for me
I also require a strong female character be in there somewhere at some point.
Supernatural finally lost me, because the female characters dropped by the wayside
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.