Depends on the situation, the age of the missing person, emotional status etc. You can do it as soon as but depending on the above is how fast the police will start looking.
Hellie- excuse prev post - submitted too quickly!! It depends whether its a child or an adult.You can report it far quicker if its a child but for an adult I think it has to be at least 24 hours.
Thanks chaps - the person is question has now turned up - it was my work colleague's daughter who is 30 but had gone out for a night out and not returned home. Her young son was so worried and she'd not been contactable on her mobile. My colleague said it was very unlike her to not get in touch so was a bit panicky. All's well though now, thank goodness.