How frequently you calibrate your pH sensor depends upon the importance of what you are measuring. If you are measuring pharmaceuticals or foods for human consumption, then follow federal guidelines, such as those provided by the FDA and Good Laboratory Practices (GLP). These guidelines require you to calibrate for each lot, batch, or shift, according to your protocol. If your practices could result in large civil fines for water pollution, you should recalibrate often and keep good records. If on the other hand, accuracy is less important, you might choose to calibrate less often; it is wise to recalibrate according to a set schedule of at least once a week. In the end, the user must determine the frequency for themselves, but calibration only takes a few minutes and the more often it is done, the better.