This depends on a lot of factors, including how you take your first sip. If you sip with air, you are altering the temperature of the liquid as you sip, as well as decreasing the volume of hot liquid in your mouth, so that the increased surface area of your exposed tongue can quickly alter the temperature of a borderline scalding liquid into a non-scalding one.
So, there is part of the answer: it depends on volume, and time in contact with the tongue.
Most coffee pots keep coffee at about 170°F - hot enough to scald your mouth.
This article, while not scientific, does discuss why people like foods at temperatures that cause scalding, and what better temperatures are. But the public wants what it wants.
Safety experts recommend that water heaters be set at 120°F because you will not sustain a first-degree burn on contact, which is important if one has mobility problems. At 120°F, it takes 8 minutes of contact to acquire a second degree burn. At 155°F, it takes one second.
So, to guarantee no scald on contact with a small volume of coffee, I would think 145°F would be about right. (At 140°F, liquid will scald in 3 seconds. If you don't chug your coffee, this seems safe enough. If you chug, it will scald.)