The presentation from Alvin Cox about this topic back in 2015 made some headlines, but was rather pessimistic and didn't really reflect the actual technical state. This has been analyzed e.g. by anandtech:
http://www.anandtech.com/show/9248/the-truth-about-ssd-data-retention
Basically, Cox based his presentation on the general specs for all SSD vendors regarding data retention. These specs require an unpowered consumer SSD to hold data for 1 year and enterprise SSDs only for 3 months. As anandtech shows
So even if it isn't as drastic as Cox had stated, everything beyond the specs is still a grey area and depends on the actual quality of the flash, capicators, storage temperature (and variations) and some other factors. Trying to keep all these variables at optimal values just to find out how far this grey area can be stretched really, REALLY isn't something I'd want to do with my backups...