Miller v. Alabama--what about high numerical sentences?

In recent years, the Supreme Court has held that automatic life without parole sentences for juveniles convicted of any crime, even murder, is unconstitutional.

Has there been any case law about long statutory terms? What about a case where a juvenile is tried as an adult for a series of sexual assaults and is sentenced to 300 years in prison? It’s not LWOP, but it really is.

What about 90 years? 50? 40? 25? What is the state of the law on this?

I knew a guy who was sentenced to eight hundred years.

IANAL, but isn’t part of the reason for 200-year sentences (or the sort) precisely that they are exempt from many of the regulations that govern life sentences, and hence are a loophole with which to lock someone up real good?