The stepper motors in the DOS are not user serviceable/replaceable. They have an MTB (mean time before failure) of 5000 hours. From personal experience and our testing they typically run much longer. If you are having issues with your DOS, then please contact our support team,
https://neptunesystems.com/gethelp or give us a call at 408.275.2205.
As mentioned above by other helpful users, when to replace the DOS replacement heads really depends on how they are being used. If they are running AWC at a high rate of flow (i.e 250 ml/min), then you can expect to replace them more frequently than someone who is running them for additive dosing and at 7ml/min. The method of checking the calibration every 6 months or so, or doing a visual inspection helps to determine "when" you should replace them.