I'm not sure there's any formal notice period: the norm is to give at least four weeks (and if you're on projects you lead on which you'll have to hand over, longer is always appreciated!). I have known people to hand in their notice on the basis of the pay period - i.e. COs giving just a week's notice, or EOs up giving only a fortnight - but that's rare; equally, I've seen people give two months' notice, even within the junior to middle ranks.