The numbers tend to vary wildly depending on the brand, size, use of wear levelling, etc. At a minimum, 10,000 write cycles would be expected with some cards claiming up to 100,000 write cycles. On the µSD card I used in my Raspberry Pi 4, it took a lot less time than I had expected to run into problems but then like most Linux OS, Raspbian was endlessly writing small chunks and the wear levelling seemed to be fairly ineffective.
One item I found useful was the LinuxPi's
The Pitfalls of Long-Term SD Card Usage in Raspberry Pi Projects. Since a Kobo ereader uses both logging and a database, it does make using SD cards for storage a bit iffy for heavy use.