There are a series of fundamental constraints on the length of observations with Herschel. There is an operational constraint that the coolers on PACS and SPIRE must be recycled for approximately 2 hours every 48 hours. However, in practice, the limit is imposed by the need to have a 3-hour daily telecommunications period (DTCP) with the ground station to download data and upload instructions every day and up to 3 hours each day are also reserved for routine calibrations. Thus there is a limit of 18 hours to individual observations with Herschel that is hard-wired into HSpot. Observers who wish to take longer observations than this must split their AOTs into shorter segments. Special care should be taken when requesting observations close to the 18 hour limit that they will remain possible even if there is a slight change in on-board observation or calibration strategy as knowledge of the instruments improves in-flight. Very long AOTs impose strong constraints on mission planning and may be difficult to accommodate in the telescope schedule because they will effectively fill an entire observing day and block it for other observations. However, for a photometric deep integration on a fixed target, the telescope can only stare at a single point in space for 50000s (13.9 hours) thus, the maximum point source photometry AOR length is significantly shorter than 18 hours.
For PACS spectroscopy observations there is a significant danger that, for very long integrations, an instrumental glitch could cause the entire observation to be lost. You should check with the PACS spectroscopy experts via Helpdesk if you are uncertain what to do to make your observation more robust against a failure of this type.
Moving targets must be dealt with in mission planning in a different way to fixed targets, as the spacecraft must calculate an instantaneous position and track on it, rather than on the stars. This requires the mission planning software to interpolate the position of the object at any moment from the Chebyshev Polynomials that define the target's ephemeris. This process may not be valid for integrations longer than 5 hours on fast-moving targets, hence the tracking accuracy cannot be guaranteed for longer moving target AORs; HSpot places a blanket limit of 5 hours on the duration of a single AOR for solar system objects.