As part of a recent AVR project, I wanted a way to increment a counter and store it even if the power was lost. This could be done with an external flash memory device, but I wanted to use the internal EEPROM of the AVR.
The internal EEPROM is limited to around 100,000 writes. Independent tests have shown this can be doubled, but 100k writes is only about 27 hours if saved once every second, so not practical for a device I wanted to last at least five years.
It takes a maximum of 34ms (8448us per byte) to write a 32 bit (4 byte) integer to EEPROM. Due to the low power requirements of an AVR, I was confident that a decent sized (say 2200uF) capacitor would allow an AVR plenty of running time to complete EEPROM writes/saves before power is totally lost.