My bad, I misremembered, the base is actually 3 days (72 hours, that's the number I actually knew, but I mixed up with something else), and can be prolonged for 10 days by prosecution if a judge allows it, and can be prolonged for 10 more days after that, again validated by a judge. After that, the detainee needs to be either charged or released. Supposedly, the extensions are for cases where there is risk of destruction of evidence or disappearance. Wikipedia cites some (old, 1987) numbers that 85% of arrested detainees were kept for longer than 72 hours and more than a third were held without charge for longer than ten days.

Interestingly enough (having now done some minimal research), for terrorism cases, you can be detained up to 28 (!) days without a charge in the UK. https://www.liberty-human-rights.org.uk/sites/default/files/...