It is recommended that all 7250 series calibrators be zeroed at least every 5 hours (at least every 1 hour for LP models) to operate within uncertainty specifications. See this article for summary specifications. QBT Product Specification
If the controller (or pressure monitor) operates in both gauge mode and absolute mode then the Quartz Bourdon Tube (QBT) sensor should be zeroed when switching modes (e.g. from absolute mode to gauge mode, or the reverse).
In gauge mode (or absolute mode with barometer option), zeroing is typically done before running each test as it typically only takes a few minutes to complete this process.
Here is an example zeroing screen in gauge mode.
In Absolute Mode with Vacuum Reference Option or Permanent Absolute Option models, zeroing is typically done at least every 5 hours. The Vacuum Reference option includes an internal vacuum sensor that is used to zero the QBT sensor. Here is an example zeroing screen for an absolute mode Quartz Bourdon Tube (QBT).
A hard vacuum is pulled on the sensor. It is recommended that the sensor be pulled down to a vacuum of less than 200 mTorr (preferably less than 100 mTorr) in order to obtain a proper zero. The Calibrator can be zeroed at higher vacuum levels; however, the uncertainty of vacuum sensors tend to increase at higher vacuum levels, therefore, the uncertainty of the calibrator would also increase if zeroed at higher vacuum levels.