No there is no magic number as degradation depends entirely on the conditions that the shoes are stored in.
Major fluctuations in ambient temperature, high ambient temperature, humidity and exposure to UV all factor in.
Your own wear pattern and body weight also affects the likelihood that something breaks down quicker.
Life is actually extended if you wear sneakers intermittently. So in a case study, if you owned two pairs of OG Neon 97s in the exact same conditions except you wore one pair a dozen times over the years while the other pair remained deadstock and untouched, the deadstock pair is more likely to crumble/disintegrate.
I personally keep to the rule of thumb that anything over 10 years is going to crumble 9 times out of 10.