This is usually arranged in advance of publishing the schedule through communication with the FACET user manager. She will request and digest your beam requests and work with you to develop a reasonable number of shifts to fulfil science goals.
If your results during a run indicate a need for further shifts, a detailed proposal on how many shifts, the goals and the analysis of shifts so far should be produced. This should be sent to the FACET User Manager who will pass it on for review by the FACET Division.
Many experiments at FACET get single shifts sporadically across a run (gaps between shifts can allow for data analyses and are usually part of a user beam request). Long programmatic studies can expect to get a series of shifts across a week or two for example. The assignment of shifts usually comes down to the science need and also the recommendations of the SAREC committee for priorities for the FACET facility.