Determining how long a patient stays in the hospital

So I was reviewing the documentation for creating new diagnoses and noticed that <Duration> was now considered deprecated. I was always under the impression that the duration number for a diagnosis was the amount of time a patient spent at your hospital. Now that I know that this isn't the case, my question is: what determines when a hospitalized patient leaves your hospital? Is it when their main symptom is cured and they have no untreated collapse symptoms remaining?

From my observations, hospitalized patients leave hospital in the morning at the same time around 8 a.m. - you get a big cash intake usually. This happens, when the main symptom is cured. Since they stay longer in hospital than a clinic patient, they are more likely to get more stuff cured but I have seen docs neglecting treatments like Ice Wraps and similar which I find annoying as there is a bonus prestige for patients "treated" so every treatment administered. This is important, if you want more patient intake or money. In case of observation ward they leave ASAP if their disease is known and cured from what I see - at least if it is a clinic-level disease.