See: http://lesswrong.com/lw/19m/privileging_the_hypothesis/

How long can they hold you while fishing for evidence?

In England the justification has to be approved by a Custody Sgt initially that gives up to 24 hours. A more senior officer has to review the continued detention from the 6 hour mark up to 24. A Superintendent or above can authorise up to 36 hours. Beyond that a Magistrate can authorise up to 96 hours in 36 hour blocks. During this time the individual has access to legal advice, where the reasons for continued detention have to be provided and the evidence thus far collected explained (again unless a very senior signs off on why not).

Unless you're been arrested for a very serious crime, in which case you're going to be defended for free by a very experienced QC who will tear apart dodgy process, you are likely to be arrested for a handful of hours. The exceptions would be being held overnight, if you're drunk/high and thus incapable of being interviewed etc.

Things are slightly different in Scotland where you get detained not arrested, but the principles are broadly the same. UK policing as a whole has been through periods of dodgy processes and huge reforms in the wake of them. There are likely to be very few criminal justice systems in the world I'd rather interact with than the UK one, especially if I were accused of a serious crime.

Edit to add - plus the premise of "fishing" implies there is no existing evidence. The initial justifications for the arrest have to be supplied at the start, and "it might have been him" won't pass muster.

> plus the premise of "fishing" implies there is no existing evidence.

I'm not sure where you get that from. I would assume you fish where you expect to find something, and my interpretation of the many places "fishing" is used (or even "phishing" to a lesser degree), is that it's done where you expect some payoff. Maybe you're thinking of the phrase "he's just fishing", which while often used by people to describe someone searching for something, often fruitlessly, generally the person searching is doing it with the expectation they will find something.

In a legal context there is the concept of a "fishing expedition" which is more of a looking for evidence when there isn't sufficient underlying evidence to justify the intrusive methods being asked for.

Ah, you meant "existing evidence" as in "currently existing evidence", while I interpreted it as "there exists evidence".

They kept me for about 6 hours. Mainly because I phoned up a lawyer and told them I wouldn't be speaking to them unless he arrived. Took the lawyer a few hours to get there.

Years, according to the New York City Department of Corrections.

It's not really fishing. They had forensic evidence for a specific crime, used it to find him (the suspect), conducted their investigation, cleared him of that crime, and let him go.

Fishing is where you pick someone first, then go looking for evidence of any crime.

It's still fishing if the forensic evidence does not justify the investigation. For example, a thumb print on a telephone book, but not on a doorknob or anywhere else that might suggest there's a reason to think the thumb print was any less than casual/innocent.

24 hours. Can apply for up to 96 hours for a serious crime. Or 14 days under the Terrorism Act