Answer
There is no rule that states that you have to wait 24 hours to report a missing person. It depends on the person who is missing, their age, mental state and vulnerability.
If the missing person is a child, elderly or a person in a very depressed state then you need to alert the police straight away.
If the person is late home or late for an appointment, then it is sensible to try and contact the person or the last place they were, then wait a reasonable amount of time prior to contacting the police.
For more information and help please see below and the websites in related information.
Missing People (registered charity No. 1020419) is the UK's only charity dedicated to finding missing people and supporting those left behind. It supports the work of police, Social Services and works in partnership with other agencies in the field.
Missing People - for sightings and to report someone missing:
Runaway Helpline - for young people under 18, to call who are away from home or care:
Message Home - for missing adults, allowing them to pass a message home: