DDRx JEDEC Standard: Retention Time
The 64 ms are mentioned in the standard by tREFI. It is usually 7.8 us, multiplied with the number of rows you will end up at 64 ms per cell. However in the new standards the DRAMs refresh several rows at the same time (c.f. https://dl.acm.org/doi/10.1145/3132402.3132419)
The reason why we have 64 ms is a good trade-off between safety and performance. However, the 64ms is also not a constant the tREF depends also on temperature. Above usually 85 degrees this changes (see datashees and standards)
In this works:

https://ieeexplore.ieee.org/document/8342023/
https://dl.acm.org/doi/pdf/10.1145/3240302.3240424
https://ieeexplore.ieee.org/document/8342249/

you can find some retention time plots for different temperatures of DDR4 technology, that we measured on real devices.