That was all a couple minutes at best, and Itachi went from keeping up with Hebi Sasuke to barely being able to walk. Its very strongly implied that using the Susanoo did worsen his physical condition badly, as it has been shown to physically affect MS users. Hell, even assuming Itachi is in perfect health, which he never really was, it just adds up to a war of friction. Totsuka doesn't have the feats of sealing a non-living being like the Susanoo, and nothing else Itachi has is even gonna dent its later versions, so its Madara biding time until Itachi is physically wasted, at which point he just finishes him off.He maintained it long enough to tank Kirin, pressure sasuke into letting Oro out, fought oro and sealed hyrdra, then approached sasuke for several minutes after sasuke desperately tried to fight susanoo with kunai and bombs.
THEN, finally, Itachi maintained his susanoo up until the very instant he died, meaning nothing said he was even out of chakra, because he is canonically stated to have died from his illness in that moment, not chakra depletion.