Unfortunately we don't know and the wording was likely selected specifically to make it ambiguous. We can surmise that it is a lengthy period because the Prequel comic sentences Dev-Em to death / execution for a single murder. Here, Zod has killed at least one Council member and Jor-El, not to mention the countless individuals caught up in his attempt coup. Even if the death penalty was abolished, as a matter of justice, the punishment would tend to scale to its severity (which is why, in our culture, we routinely ascribe punishments which cannot possibly be served by the convicted)... of course, that assumes Kryptonian culture has that kind of sense of justice and does not simply do the most utilitarian thing (implied by the usage of the Black Zero and its complicated customs).
I would tend to believe Kryptonians age slower after reaching maturity. Zod indicates that they had been freed and wandering for 33 years. Neither he nor Faora aged appreciably in a manner that a human woman at maturity would if you added 33 years. This can be explained either by slow aging or by shifts in hibernation, but I'm inclined towards the former because of Krypton's mastery of eugenics and the great age and stagnancy of their society which makes a certain amount of sense for long-lived beings for which time slips by quickly without realizing it. It is a common trope in fiction that the aged tend to be more passive in their nigh immortality.
As for the gravity of Krypton, doubtless it is greater as it is explicitly stated in the film, but not magnitudes greater. We see objects fall and can, more or less, measure the gravity of their planet which- surprise surprise, is basically Earth's. It's easier to reconcile Jor-El's one line than it is to try to make high gravity work across Krypton given what we see throughout the film.