The only reg I can find which seems to address this for Part 25 is 25.831.
14CFR25 SubPart D - Centilation and Heating
The only explicit temperature reference is 25.831(g) which describes conditions after an improbable failure, and allows up to 35 deg C INDEFINITELY in such a case. That's pretty hot. (But it's not normal ops, obviously)
More likely, 25.831(a) applies:
(a) Under normal operating conditions and in the event of any probable failure conditions of any system which would adversely affect the ventilating air, the ventilation system must be designed to provide a sufficient amount of uncontaminated air to enable the crewmembers to perform their duties without undue discomfort or fatigue
But unless there's an AC or other interpretative material out there, that's just a qualitative standard.
AC 121-35 recommends deplaning
pax within 30 mins of a ventilation failure ...