The legal minimum in most of the United States is 2/32 of an inch, but grip in the wet falls away long before that. Many drivers change at 4/32 for a meaningful safety margin in rain.
Check depth with a gauge, or use the penny test: insert a penny with Lincoln's head upside down into a groove. If you can see the top of his head, you are at or below 2/32 and the tire needs replacing.
Tread is not the only reason to change. Replace a tire if you find sidewall bulges, cuts that reach the cords, repeated slow punctures, or vibration that balancing does not cure.
Uneven wear is a message rather than a verdict. Wear on both shoulders usually means under-inflation, wear down the centre means over-inflation, and wear on one edge normally points at alignment.
Replace in pairs across an axle at minimum, and ideally all four together so grip stays balanced.