♥ Hugging You, that's what friends do!~ ♥
Usually lengthier anime put me off. I feel like 3 or 4 seasons is usually more than enough to convey a long story. Most shows get stale after that point and don't entertain the same way. It's why, sadly, I really can't get into long running shows like Bleach, Naruto and One Piece (even though this one was decidedly awesome in the first 4 season/arcs I bothered to watch).
I do make exceptions for good shows, and some shows are good enough that their length is irrelevant due to their inevitable variety, like Star Trek. But not all mediums of television are forgiving of repetitiveness, and animation just so happens to be one of the least forgiving when it comes to staleness.
My Little Pony: Friendship Is Magic, is quickly approaching the "typical point of staleness" so I will soon see (December I believe) if the series continues on with the same originality or not. I could definitely see myself departing the fandom at S4 if it is extremely poor compared to other seasons.
This leads me on to my next point. The "Beginning" exception. I make exceptions for series that are newly assigned a long running status if I am currently watching it as it is released to the public. I find that usually the release rates are slow enough that they don't make demands on my time, so I don't have to allocate days or weeks to "catching up".
But yes, the "Catching up" problem is a huge one for me. If I cannot reasonably catch up in 1-2 days, I usually drop the show or consider myself too far behind, regardless of it's long running status.