Herschel is designed to provide at least three years of Routine Science Phase (RSP) observations after the initial mission phases including the Science Demonstration Phase (SDP, see the Herschel Observers' Manual). The current best estimate of the mission lifetime still has a significant uncertainty, being in the range 3.5-4 years, but providing confidence in achieving three years of RSP operations.

The total available observing time to be allocated to GT and OT observers in the nominal three year RSP is 19,776 hours. This figure is obtained from 21 hours schedulable observing time per day for 3x365 days, minus 14% which has been reserved for observatory calibration and engineering observations; i.e. (21 x 3 x 365) x 0.86 = 19,776 hours.

Although there are many conceivable reasons for why the actual amount of available observing time may differ from this number, in both positive and negative sense, this nominally available observing time is adopted for conduction of the AO process.