Depends on the anime...I pretty much dropped off Inuyasha about 60 episodes in because of the pacing and I stayed with Dragonball Z until the end of Cell Games (I taped the Buu Saga, but haven't come remotely close to rewatching any of it). They just go on too long sometimes and in some cases, the filler episodes border on irresponsible. Others though, like Tenchi Universe and Outlaw Star could have benefitted from longer runs (52 Episodes).

If it's more of an episodic show (as opposed to one where you have to have seen the previous episode to know what's going on) and I like the characters and scenarios, they can keep milking it. Galaxy Angel and Ranma both reach well over 100 episodes each, through various seasons and they feel short to me because they're so enjoyable. Same for Conan, though I haven't seen anything past the Adult Swim run. Tenchi Universe was wasted potential in this sense...They could have made a fairly enjoyable sit-com with the cast and milked it for a good while, before going to the outer space adventures.

Yea, it really depends on the show for me, but I guess I would have to say I like the 13-26-52 length ones that have atleast some plot and story development in most episodes. I don't really like the ones that have around 100+ episodes where each episode is just an entire action scene or hardly anything ever happens.

13 is good enough for me. It's harder to do but I think it's worth it if you can pull it off. Since I don't read much manga anyway it doesn't really matter if they didn't put in an "essential" character because I wouldn't even be aware of it for it to ruin the show for me.

I only tend to be concerned with 13 episode series if the story seems rushed or the plot is too different from its manga counterpart. I may go back to the read the manga only after watching the anime.

100+ as long as theres some new conflict and or resolution going on between the characters or storyline, its fine with me.