Google pays annual performance bonuses as a percentage of base salary. Target bonus by level:
L3: % target. L4: % target. L5: % target. L6: % target.
Actual payout depends on your performance rating: Needs Improvement (%), Meets Expectations (% of target), Exceeds Expectations (+), Outstanding (+).
Most engineers get "Meets" or "Exceeds." The bonus is paid once a year, typically in March.