Your tires have wear bars on them and they should be replaced when you get to the wear bars. At that point they do not have enough tread left to displace water and could cause you to hydroplane on water.
If you're unsure if your car is ready for tires stop by and we will check them for you and let you know if you're due. I would also suggest an alignment whenever you get new tires. It's protection on the investment you made on your car.