It will definitely be shorter than if you were able to store them at cooler temps with lower humidity. How much shorter? I really can't say. I did some Googling and did find this if it makes you feel any better:
https://www.verbatim.jp/download/produc ... ake_vF.pdf"The discs were stressed in a combined temperature, humidity, and light cycle (Section 1.2.2, p.3). The discs
were subject to the following test conditions in the environmental chamber: 85°C, 85% relative humidity
(conditions specified in ECMA-379) and full spectrum light (per MIL Std. 810G) (Figure 1-1, p.3). The test
was repeated three times with identical results."
and
“None of the M-DISC™ DVD media suffered any data degradation at all. Every other brand tested showed
large increases in data errors after the stress period. Many of the discs were so damaged that they could
not be recognized as DVDs by the disc analyzer”