The answer to this depends on one’s philosophy of what the prison system is for.
For those who feel that the prison system is there for punishment and deterrence, we do not lock people up for long enough. To people who believe this, criminals would be less likely to commit crimes if they knew they were likely to be sentenced to long terms that they actually had to serve out. To such people, then, our sentences are too lenient.
For those who feel that prison should be about rehabilitation, we lock people up for too long, particularly on minor crimes. To this school of thought, prison terms only make people more likely to be criminals when they get out. Prison does not help inmates but, rather, makes them into hardened criminals who can do nothing but commit more crimes upon release. In this view, we keep people locked up too long and we do so without helping them avoid committing more crimes when they are released.