Use millis(). Save the value from millis() in a variable (use an unsigned long variable). Then in the main loop, constantly check millis() and compare it against the saved value. When it's 25 more than the saved value, then 25ms have elapsed, so do the next write. The timing should be reasonably accurate, within half a percent or so.
You are aware that according to the datasheet, EEPROM endurance is 100,000 write/erase cycles? At 40 writes per second, that works out to 41 minutes or so. Some folks have tested this and reported significantly higher endurance, but once the datasheet spec is exceeded, I wouldn't bank on it.