I need to expose a Raspberry Pi 3 to high temperatures, how much is okay?
The place of production is very hot (I do not know how much still); before installing it I wanted to know how many degrees it can handle.
As noted in the Raspberry Pi FAQ:
The Raspberry Pi is built from commercial chips which are qualified to different temperature ranges; the LAN9514 (LAN9512 on older models with 2 USB ports) is specified by the manufacturers as being qualified from 0°C to 70°C, while the SoC is qualified from -40°C to 85°C. You may well find that the board will work outside those temperatures, but we’re not qualifying the board itself to these extremes.
The Pi 3's SoC has been widely reported as suffering from overheating issues; exceeding 100 degrees Celsius in testing, at which point the CPU is heavily throttled. That said, it does show that the CPU is generally capable of surviving these temperatures for a short time.
For long-term operations, though, operating near the upper end of the spectrum (probably > 70 °C) risks damaging the board. Note that the SD card might actually be the weak link here; SanDisk only promise 28 hours of lifetime at 85 degrees Celsius:
Temperature proof: Capable of withstanding operating temperatures from -13 ºF (-25 ºC) or 185 ºF (85 ºC) for 100 cycles (equivalent to 28 hours).
Poorer quality cards probably won't even manage that, so a heavy load on a hot SD card will likely lead to it suffering a premature failure.
Below 50 to 60 degrees Celsius, there should be few concerns, and most components should last a reasonable lifetime. Above this, heat sinks and cooling mechanisms would be a wise idea — there are some ideas here. Short periods at extreme temperature around 85 ºC may be manageable, as long as the exposure is not for a very long time.