Say that a Fire Pokemon (like a Charizard) is placed inside a box which is heated until it reaches a theoretical infinity. At what point does it 1.) feel pain 2.) burn 3.) die. If anyone has any canonical feats that would answer this, that would be great.
I can't think of an example of maximum temperature but we can establish some baselines;
In the original Pokemon Red pokedex (for the Gameboy) it stated that:
Charizard, the Flame Pokémon. Charizard's powerful flame can melt absolutely anything.
Taking this at face value, (and given that the element with the highest melting point is Tungsten) this would be 3500°C.
In one episode of the anime series (Volcanic Panic), Charizard and Magmar fight. Both end up covered in molten magma (approx 1000°C) and both attempt to flame each other with flames that are described as being similarly hot, with no effect.
In "Some Like It Hot!" (Magcargot! Getting the Hot Heart!!), Ash and crew encounter the Magcargo. The pokedex entry on Bulbapedia describes it thusly:
Magcargo's body temperature (18,000°F) is much higher than the temperature of the surface of the sun (9,940°F).
When Ash encounters one, there's a brief discussion of which Pokemon he could use to attack it. Sensibly speaking, we can assume that Charizard would be able to resist it since Ash doesn't mention its weakness to the temperatures it can generate.
Unless they have an ability such as Flash Fire, fire Pokémon take damage from all direct fire attacks.
The weakest fire Pokémon we have numbers for (that I can find) is Magby, according to it's Pokédex entry it exhales flames of 1,100 degrees F. Comparing stats, Vulpix has a lower Special attack stat than Magby and so likely has a weaker flame still, not to mention the non-fire types who can learn fire attacks. As Magby is the weakest we have numbers for however, it's a good benchmark.
The lowest temperature flame that we know almost all fire Pokémon take damage from is Magby's 1,100°F (593°C) Fire Spin attack. (In the video games)
Well, going on the Pokemon Red Pokedex entry for Charizard, if its flames can truly melt anything based on that entry, it would, AT MINIMUM, have to be 3942 °C, or 7128 °F, the temperature for the compound with the highest melting point, Tantalum hafnium carbide (Pronounced: TAN-tal-um HALF-knee-um CAR-bide). Going for the ELEMENT with the highest melting point, Tungsten, Charizard's flame would have to be at least 3,422°C, or 6,191°F, which is hotter than THC. If anyone can find something higher than that, I'm a potato.