We do know, somewhere between not long enough and plenty long enough. There are three estimates of lifespan on an SD card. It is the same as cars and home appliances. Depending on individual use, while we don't expect them to break, they do but different components fail at different times. Consider that you and your neighbour both buy identical brand new cars on the same day. Then you go about your daily lives. At some point something will break on the cars but it is highly likely it will be different parts at different times. Same with SD cards.
On Average....
Time based: 10 years lifespan
Electrical usage: 1m - 2m hours
Write cycles: 10,000 per block
Remember an average. If you have 2 cards, one fails on day 1, the other fails in 10 years. Average is 5 years.... reality is you have a 50:50 chance of it going wrong sooner.
The point stands, dont rely on them alone for multi year archival purposes.