The trouble is, it's such an arbitrary number. Do the police claim that if given the 90 days they will increase their chances of stopping any terrorists by xx%? Of course not. Would only 45 days halve those chances? Who knows. I'd happily give them 6 months, if they can guarantee me results. But they can't, can they? And instead I am just running the risk of disappearing for 90 days the next time I return from Greece suntanned and run for the bus.

So - sell me a number of days, but give me guarantees; otherwise you just look inadequate.