It's long in the context of how much mental health treatment they can have provided him over that time.

If 7 years is how long it took to be sure he wouldn't reoffend, why go longer? It would be a waste of taxpayer money, it would worsen his life for no benefit, it wouldn't solve anything for society, and it wouldn't even be useful as a deterrent because I'm pretty sure he wasn't thinking about ROI of crime vs. potential punishment when committing the crime considering he believed God was ordering him to do it, and he didn't deny having done it.

If mental health treatment were so far advanced that his problems could have been 100% cured within a month, I would have supported his being released after a month.