28 days would be about the right time for Blair to make his excuses and leave.

As that article I linked to mentions, up to now the police haven't actually used the powers they already have in dealing with terrorist threats. This is essentially political sleight of hand, designed to distract attention from the failures to date, and to give Tony Blair a chance to grandstand, and to face down the people in his own party who want him to go. It hasn't worked in either respect, it appears.