Humidity and temperature are not the only things affecting lifespan of optical discs. The quality of the original materials used in manufacture, the quality and speed of the burn, and proper handling and protection of the reading side of the disc also come into play. Deterioration of the reflective coating can also cause issues.
So, you cannot narrow down the possible issues to just humidity and heat. UV radiation is another destructive element, though you'd have to expose the discs to sunlight for quite a while.
That said, I'd prefer not to store discs in any area that regularly reaches 35 C Â°. Twenty-five to thirty should be ok.
I have many CD's from 1999-2001 era that are still perfectly readable. I recently weeded out a bunch of those, all Taiyo Yuden discs. And I have many examples of DVD's from 2003 that are perfectly fine also, but I always bought high quality discs, like Taiyo Yuden, Verbatim and Japanese made Maxell.
The oldest Blu-ray I possess only go back to 2008 or so, but still holding together well. The Ritek discs went bad in three or four years, but the Verbatim and FTI discs are fine.