Itachi had his susano'o activated for over an entire chapter after already exhausting himself. Not to mention that was when he was terminally ill, which isn't the case in most scenarios you'll find on NB. If you want the entire rundown on his chakra capacity, then read for yourself. It isn't by any means low.
> Before his battle with Sasuke Itachi sent out two Karasu Bunshins [Crow Clones] to meet both Sasuke and Naruto [
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> Itachi, not taking Sasuke serious yet, used genjutsu three times in a row as the fight commensed [
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> Itachi then used another Karasu Bunshin [
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], followed by a shunshin blitz [
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> Used one his chakra taxing MS techniques, Tsukiyomi [
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> Used the Katon: Gōkakyū no Jutsu [Fire Release: Grand Fireball technique] not once, but twice in a row [
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> Itachi then proceeded to use his second MS technique, Amaterasu [
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] to only follow back up with using it again, in quite an alarming succession [
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] which practically lit up the entire Uchiha hideout [
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> Shortly after straining himself by using Amaterasu twice in a row, Itachi is forced to throw up his third and final MS technique, Susano'o, in order to protect himself from the destructive power of Sasukes' kirin [
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> Itachi decided to throw up the final version of his susano'o armed with the two legendary weapons [
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] and then proceeded to battle Orochimaru's biggest transformation, Hydra, and sealed it away [
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]. Overall, Itachi had his susano'o activated for more then an entire chapter before collapsing; which should stand for more or less 4-5 minutes.
> And to top it all off, while Itachi had already overly exhausted himself, he still managed to use a transcription seal [fuinjutsu] and sealed away one last Amaterasu within Sasuke [
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