When is betta tail rot is too advanced?

Betta fin rot is never "too advanced". There are three things that you will use to treat all fin rot, no matter the severity: clean clean water, warmth, and proper food. If the tank is unfiltered, change the water daily or at the LEAST every other day. This keeps more bacteria out of the wound and lets new fin tissue regrow. New fin tissue will be whitish or clearish and very thin and wispy at first.

Secondly, warmth. Bettas are a tropical fish and cool water lowers their immune system. If his water is not at least 72* F, he is too cold. Ideally he should be around 78* F. You must either move his tank to a warm part of the house or get him a heater. Your local pet store should have a small heater for bettas. If you don't know what temp his water is, you need to get him a thermometer too.

Thirdly, proper diet. Ensure the food you're giving him is formulated for bettas. They need more protein than basic "fish flakes" will provide since those are formulated for plant eaters, and the betta is a little carnivore. He will need that protein to regrow his fins.

You may wish to add some aquarium salt too, to aid healing.

If the rot is very advanced (ie, less than a half-inch of tissue left) then you may need to go to your local fish store and buy an antibiotic for fish. This is in addition to the steps listed above.

If you do not treat promptly, this can spread to the betta's body (and is now properly called body rot). At this point, his body will begin to be eaten away by the bacteria and it is probably "too late" to save him yourself. This is why it's important to always keep a betta's tank super clean to begin with to prevent this from ever happening.