It varies from product to product...
However Stryker's points are extremely valid.
I reside in singapore, and while my Galaxy Soldier(released in 2018) has melted into mush last year. My brown non-metal body(released in 2014) which am using for my custom slave leia is pretty much still intact with no lose in quality.
The galaxy soldier was very much unprotected against the elements and it was also under sunlight. Whereas Leia was kept in a showcase, and away from the sunlight in a monitored humidity room.
I didn't apply powder nor oil on any of my tbleague bodies thus far, and kept them mostly in museum poses. However, I noticed that constant moderate play in posing seems to reduce the breaking or tearing of the silicon body, as compared to those posed once, then reposed again after a year or two.
Still despite these, am still buying these tbleague bodies, because they certainly are still the pioneer and the better quality ones out there. (Lost count on the number of them i have.... think i have over 50 bodies now? Due to resident evil, final fantasy, king of fighters, street fighter, ghost in the shell, gundam seed, star wars etc, customisations)