Complex sentence: Managers reportedly have been forced to work 80 + hour weeks on a regular basis to as much as 120 hours in a single week for three weeks straight during the time known as "year-end" in the weeks immediately following Christmas.
Split into simpler sentences: Managers reportedly have been forced to work 80 + hour weeks on a regular basis to as much as 120 hours in a single week. This would occur during the time known as "year-end" in the 3-4 weeks immediately following Christmas.