How often is data transferred from RAM to EEPROM ?

It depends. EEPROM stands for Electricaly Erasable Programmable Read-Only Memory. The first EEPROM were erased at once (whole content). Later, smaller regions of the memory could be erased. And the latest can even erase individual bytes. Meanwhile Flash ROM appeared -- it is similar in principle, but it is designed to erase bigger chunks of data. On the other hand, it is better in storing the data (capacity). Also Flash ROM is usually faster for reading and slower for erasing/writing. But "Flash ROM" was somehow different from what is understood as FLASH memory now. (The Flash ROM use the NOR flash technology is more reliable, faster for reading much slower for erasing). The FLASH (as in memory sticks; NAND flash technology) is also the type of EEPROM. It is much faster than older EEPROM, has bigger capacity, but data is accessed in large blocks, and is less reliable than Flash ROM.

Historically:

- ROM was manufactured with the content inside.
- PROM was programmable -- manufactured as blank, and programmable only once in the special device.
- EPROM is PROM that can be erased (Erasable PROM) using UV light (and then programmed again).
- EEPROM is Electrically Erasable PROM -- the acronym is rather old, but it is used for variously modern implementation of the principle.

This way, you should specify better what type of EEPROM you talk about ;)

The EEPROM data is cached in RAM.  My question is typically how often is data transferred from RAM to EEPROM on every key cycle?