Complex sentence: The Mods examination has a reputation as something of an ordeal: it evolved in the 21st century from 11 or 12 three-hour papers across seven consecutive days into 10 or 11 three-hour papers across seven or eight days.
Split into simpler sentences: The Mods examination has a reputation as something of an ordeal. It has changed in the 21st century from 11 or 12 three-hour papers across seven consecutive days into 10 or 11 three-hour papers across seven or eight days.