In the end it all depends on the story.
Some stories are perfect with only 13 episodes while other might need 26 episodes or more.
I honestly don't care about the "length", the most important thing is how you utilize that "length". (Double entendre not intended)
Some good examples
Madoka Magica, had 13 episodes but it was enough show us all the events without adding more fluff.
Haruhi no Shoushitsu, it's a 2 hour length movie but how the story was "Adapted" that made it perfect
Steins Gate, had 24 episodes, but it used it builds ups and climax points perfectly.
Some Bad examples
Gundam Seed Destiny, had 50 episodes, but most of it were recap episode retelling 2-4 previous episodes.
Fate/Stay Night, it had 24 episode, but the problem lies within the large amount of scenario not able to be retold in this media, hence it fails in many aspects compare to the VN
Shingetsutan Tsukihime, it had 12 episodes, and It does not exists (same reason with fate/stay night)
shonen and shoujou animes are out of the question, since the episodes are made by demand.
-Bleach, Naruto, One Piece
-Shugo Chara, Pretty Cure
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