There’s alone and there’s alone in a small room designed to be a form of sensory deprivation when you’re in a high emotional state and lack self control.
15 days is well into brain damage long. That’s 15 days of 0 stimulation in a small box. Generally speaking you should see another human face every few days minimum
4 hours is good for the maximum and I’d hope they case by case give 1 or 2 hours. 4 hours in solitary doesn’t have to mean “after 4 hours you have to be let out” it can mean “after 4 hours someone comes by, checks on you and gauges your emotional state via conversation”.
I could also be ok with situations where they have access to positive stimulation (like books) for more than that time. But that gets into the realm of “it isn’t punishment, it’s alone time to calm you down”.
Overall our current prison system needs an overhaul led by psychologists. It isn’t giving us pro social results. It takes people who are overwhelmingly some variant of mentally ill, torturing them while sticking them with other criminals, and then plopping them back on the street with trauma, no life skills, and debt. Many of these people were never free adults when released the first time.