Hi! I work as a physical therapist in the U.S. The wording of your question leads me to believe you are not from the U.S. So, if this does not apply to you, just disregard. If you are from the U.S., your wording is not like any I've heard before, and seems off. We would never call a patient an inmate.
Part of the answer would depend on why the person has diplegia. The most common cause is cerebral palsy. In this case the person would most likely live with their family. If they were in therapy, it would most likely be outpatient, in which case they would travel to the facility, usually 2-3 days/week, for a period of time.
With insurance, probably 4-8 weeks total.
If the person sustained a SCI which resulted in diplegia, they would be hospitalized initially. In the hospital they would likely have some form of rehab once they are stable. When the person is ready for d/c from the hospital, the team decides what is next. The usual choices include inpatient hospital rehab, skilled nursing facility, home with home care therapy, home with outpatient therapy, or perhaps home with no therapy. The patients are not divided by diagnosis, although there are some hospital facilities specializing in SCI for example. The patient's length of stay would depend upon many factors.
If the patient has inpatient rehab, most often there are 2 people in each room. Sometimes, you get a private room, but not usually. Same thing at the skilled nursing facility. If the patient has inpatient rehab, afterward they could go home with home care, or home with outpatient rehab.
Also, I'm not sure if you are confusing diplegia with paraplegia.
If you provide more specifics, I can answer more thoroughly.