It's actual Pokedex entry says that it is 18,000 degrees Fahreinheit, or almost twice as hot as the surface of the sun (which is also the coolest part of the sun).
Now, people reasonably wonder, well, why are things not burning or melting around it? Well, we've seen several times that Fire-Type Pokemon can control what they effect with their temperature and flames (for example, there was a Ponyta in the anime that burned Ash because it didn't trust him, even it's normal rider could ride it fine, and there was a more recent example in one of the Poketoons where a Macargo could essentially turn its heat on and off to heat or cool its Trainer's house).