Never had an SSD fail so far. Do you mean that they reach the end of their wear-level as shown by various SSD-tools, for example Intels and Samsungs bundled tools, or that they just die for some malfunction reason?
They typically die for other reasons.
There have been many different writeups, some are quite comprehensive.
This one from Tom's Hardware back in 2011 covered data released by Google and SoftLayer, and from technology analysts. They basically had many hundred thousand drives they could watch and monitor over the course of five years.
This graph probably describes the rates better than any other:
That is the annualized failure rate, looking at constantly-used drives in big datacenters like at Google. Some are used continuously, others used more sporadically, some used 24/7, others used only a few hours per week. Thousands of drives in lots of situations across a wide range of usage patterns.
Note that these were corporate server failure rates. You almost certainly use your drives much less, so extend the graph dates by a few years.
The graph describes that a small number of drives arrive just fine, but then about 3% fail within the first three months. Then another roughly 2% fail between 3-6 months. Then another roughly 2% fail between six months to a year. By the end of five years, about 40% of these big drives have failed.
They can die for any number of reasons. When drives warm and cool parts expand and contract, the boards and circuitry may not handle the expansion and contraction; maybe it causes something to crack, maybe something cools too fast or too slow and expands wrong triggering a short. Maybe something overheats. Maybe they are hit by random radiation that kills a circuit. Maybe it was a subtle manufacturing defect that took a small amount of wear and tear to uncover.
Their survey discusses that specific vendors are more likely to have better drives and others are more likely to have worse drives, certain models are known for reliability and others are known for rapid failure, but ALL models from ALL vendors had unpredictable failures. Some were very small, about a quarter of one percent AFR, but that is still not zero; that is one in four hundred that dies young.
No matter the reason, no matter the brand, a small number of drives die early. Some drives fail within weeks. Other drives can run continuously for a decade without problems. But sooner or later every drive will die. You don't know when that is. You need to be prepared for it.
Personally I don't trust a work disk drive at all after three years, and I don't trust my home's disk drives after five years. At that point they are due for replacement.