The G602 runs on disposable AA batteries. Two Duracell (i.e. good, not ultracheap no-brand) batteries are included.
Logitech claims up to 250 hours in 'performance mode' (high-speed polling for gaming), and up to 1440 hours in 'endurance mode' (normal wireless-mouse performance for day-to-day computing).
Those numbers equate to:
Performance mode: 62 days at 4 hours/day, 31 days at 8 hours/day, 10 days at 24 hours/day
Endurance mode: 360 days at 4 hours/day, 180 days at 8 hours/day, 60 days at 24 hours/day
There's an on/off switch at the bottom, which based on the battery life for 24 hour/day runtime, we'd recommend using.