I wish there was more on this stuff I really do ;_;
My guess has always been somewhere around a day, maybe two or three if they have extremely good discipline. It’s a mechanism meant to keep them in line. The need is constant, but plenty of the canon characters seem to be at least capable of basic human interaction for several hour intervals (and there’s even a mentioned example in-game of one lying down to take a nap), so it’s clearly not something that would debilitate them the second they left the battlefield. But it’s always on the back of their mind and only gets worse the longer they go. I think going weeks would probably drive one to madness, if not kill them.