The JWST instrument overhead times are adopted by the Astronomer's Proposal Tool for each overhead activity, 

The Astronomers Proposal Tool (APT) charges users times and overheads for their observing programs (see overview and summary) to provide a resource accounting for each visit as well as for the proposal as a whole.  Instrument overheads are charged for operations performed during the use of each instrument.  The actual overheads applied in APT may be adjusted over time as more is learned about real operations, but representative values are shown for reference in Table 1. Further details for each instrument are provided in Instrument Specific Overheads.  Details for your observing program are available by exporting the times report from APT.

Table 1. Typical instrument overheads (in seconds) for common operations