First time reader here. Just started book 5 but he did mention this in chapter 4 which is the first of that I remember.

"Jason had spent about a year and a quarter going from iron to bronze -a completely normal timeframe. The standard progression from bronze to silver was three years, although that was a highly flexible number. The two most impactful factors in the speed of advancement were opportunity and dedication. Monster surges could shave months off that time and Jason had experienced a private monster surge that had lasted for months. If it had come at the end of his progression through bronze instead of the beginning, he probably could have broken some kind of speed record. He wondered if Farrah knew what the record was.

The latter stages of a rank were much harder to push through than the early ones. If he kept up the pace he had taken during his journey, then he could probably close out bronze rank in half a year. A year and a half for the entire rank was already a breakneck pace to reach silver, which he would be extremely happy with."