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It will vary based on several variables as the key will switch back and forth between active an passive modes. I've had my keys for almost a year and never changed the battery.
keyfob batteries on prev. cars (without the "intelligent" feature) have lasted longer than 7 yrs.
So I won't worry too much about it (passive RFID is not a huge draw anyways) - should go 4 or 5 yrs easy.