2-post lifts with a capacity less than 12,000 lbs. require four (4) inches of concrete at 3,000 psi.
2-post lifts with capacity at or above 12,000 lbs. require six (6) inches of concrete at 3,000 psi.
Check product specifications/capacity to determine requirements. Most garages have at least 4 inches. You may need to drill a test hole to measure the thickness of concrete before installing your lift for optimal safety.