What do you think madara means when he saysEdo's only have the capacity in their real lives. Itachi can't camp in his Susanoo when he was alive so he wouldn't be able to due so while in Edo. His capacity that we know of is roughly 5 minutes in V3 from when he fought Sasuke. Is this his true limit? I have no idea. He could probably hold it longer but I can't just assume he can hold it for more than what he has shown. This is why the "sick Itachi" argument is no relevant anymore. We all understand he was sick but we cant just make silly assumptions (like saying he can camp in Susanoo all day) and give him more power than he may or may not have. Point is, we don't know so we gotta work with what Kishi gave us.
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Edo's just don't run out of chakra or their chakra (like their bodies) regenerates at an extremely fast pace which makes it seem unlimited, itachi won't run out of chakra no matter how long he holds susanoo up for. There's a huge difference between 'sick itachi' and 'edo itachi'. Sick itachi started to huff and puff after using just one tsukuyomi, and he was on his knees coughing up blood after using 2 amaterasu. Even in part 1 where Itachi wasn't so heavily affected by his illness, he was huffing and puffing and had to deactivate his MS after using 2 tsukuyomi and 1 amaterasu.
Now compare that to edo itachi who spams all levels of susanoo, uses at least 3 amaterasu, 2 tsukuyomi, and izanami in a matter of hours, yet he doesn't show even the slightest signs of exhaustion (he's never out of breath) or the negative physical affects of using the MS. Edo itachi can clearly hold his susanoo and use MS techniques incomparably more times than his real self, and considering edos were stated to have unlimited chakra, he most likely can just hold a V4 susanoo as long as he wants.