The duration limits for all AORs are currently 18 hours for any Herschel observation as observatory maintenance and the need for a daily uplink means that longer AORs are not practical. If the estimated duration (not including the slew overhead) of your AOR is longer than this time, the software will return a message telling you to make the observation shorter. In general, AORs longer than about 9 hours (half an operational day) may be more difficult to schedule.

If requesting an AOR with a duration close to the maximum permitted, you should be aware of the danger that when the in-flight performance of Herschel is known, AORs that are "just possible" based on the best time estimate pre-launch, may become impossible after launch if the overheads or integration times for a particular instrument or configuration have to be increased.