Based on what he had been told, the universe would mature in three phases. The first phase would last anywhere between 500 to 1000 years, and he wasn't sure about the next two phases but it felt safe to assume that they would, at least take the same amount of time.
But Lex didn't feel like making any assumptions as that's how mistakes would happen, so he decided to find out the exact details at some point.
While that happened though… there was nothing to stop him from aiming towards having his realm mature in 3000 years, give or take. He didn't feel any real pressure to succeed though. Sure, supposedly it wouldn't be possible for a realm to mature once the universe entered its next phase, but it had already been established that there was nothing that was impossible.
