14, 28, 42, 90, ... days
The real reason for ever longer detention is just a propaganda stunt for failed governments. On a potential bad news day, the government can just send out the menacing armed balaclava clad anti-terror police for some nice big high profile raids, rounding up dozens of dark skinned individuals in a couple of northern cities, plus a quiet home counties suburb for maximum hysteria. They'll be plenty of leaked briefings about ricin, sarin, TATP, airliners and maybe even a dirty bomb, as there is nothing like a bit of nuclear to put the willies up the general public.
Trouble is when you have to let them all go again because you can't charge any of them even with the dozens of anti-terror laws, such as "suspicion of conspiring to read material which could be interpreted by Daily Mail readers as being useful in the preparation of a terrorist act", it tends to result in a bit of bad publicity. However if you can hang on to them for several months, that will have given you time to see if you can get them on any immigration irregularities, benefit frauds, tax avoidance, or failing that, turn up a bit of kiddie porn on their computer, or even better, the latest catch all with a definition so wide you could drive a bus through it; "extreme porn".
When someone finally remembers asks; what about all those people that were arrested a few months ago? The government can assure us they are all safely locked away for a range of heinous crimes. But of course there are many more out there just waiting at the even severer end of server, so it's necessary to have 90/180/365 days detention without trail.