What is it called when you stay at hospital? [closed]

Consider hospitalize ("to place in a hospital as a patient"). As in:

I was hospitalized for two days until the doctor discharged me.

Other options

I received five days of inpatient care.

My convalescence, in hospital, lasted five days.

(echoing @coleopterist)

A hospital "stay" is very common. In healthcare, we refer to the number of days that a person is in the hospital as the "length of stay."

Another antiquated term is "convalescence," though one could convalesce at home. This is the sort of terminology my grandmother would use when writing a letter -- talking about how she went to visit some friend who was convalescing at the hospital. This refers more to recovery than any other part of a hospital stay -- for example, if you are admitted for a surgery, and then are transferred to a recovery ward, your stay in recovery would be described as "convalescing".

The act of actually entering the hospital as an inpatient and starting your stay is "admitting" or "admission". The act of leaving the hospital is "discharge."

It's important to note that these terms as I've described them are specific to the US healthcare system. British English -- or other forms of Commonwealth English -- may have other terms or phrases to describe similar things. (I work in US healthcare, so I cannot make any assertions as to how the British may or may not refer to things.)