Depends on the type of flying being done. A 9am start with a 3.5 hour leg out, an hours break for lunch and then return, done 5 days per week will give 150 hours per month and relatively little fatigue.
You could also have a 50 hour month and be highly fatigued if you threw in early starts, several short sectors per day and a poor mix of early/late duties.
Twin Otter operations tend to involve frequent short sectors so yes 120 hours of this can be very tiring.