The law in most states is between 300 and 350 feet. Personally I do it as soon as I can see the tail lights simply because it bugs the heck out of me when someone comes up on me with their bright lights on.
at least 2m
500 to 1000 ft.
according to Florida handsbook, you must dim your bright headlights within 300 feet of the vehicle ahead.
If you are a courteous driver, you will dim them when you see their tail lights. No one likes bright lights in their mirrors, reflecting into their eyes.
If you are driving with your bright lights on and can see an approaching vehicle, you should turn off your bright lights immediately.
Although the lights should be dipped when approaching another vehicle so they aren't bright, the best place to look would be at the edge of the road on your side; you will still be able to see most of what is on the road ahead of you, but without looking directly at the bright lights.
When a vehicle with bright lights is approaching, you should avoid looking directly at the headlights, as this can impair your vision. Instead, focus on the right edge of the road to help maintain your lane. If necessary, reduce your speed and be prepared to pull over if the oncoming vehicle is too close. Using your vehicle's sun visors can also help reduce glare.
500 feet is what I was taught but different areas have different rules. You may have to check with the DMV or the PD about this.
When an oncoming vehicle's headlights are too bright, you should look toward the right edge of the road to avoid being blinded. This helps you maintain your lane while reducing the glare from the headlights. Additionally, avoid staring directly at the headlights, and if necessary, slow down until the vehicle has passed.
If you can see the oncoming vehicle then he can see you. Dim your lights at that time.
It perfectly depends on where you live or drive. You should dim your headlights right when you see another vehicle in front of you or right when you see the lights of another vehicle. It doesn't matter if you are seeing the tail lamp (both of you are traveling in the same direction), you should dim the lights or else it is going to make difficulties for the driver in front.
Highbeam headlights should not be used within city limits when there is oncoming traffic or when driving behind another vehicle, as the bright lights can blind other drivers and create a safety hazard.