Justbelowcap,
Is it specifically written anywhere? Maybe, maybe not - a regulator wouldn't go near it, simply because it is a function of each aircraft's systems and their associated limitations and operating procedures. And they don't have to because they already have a rule that says comply with the AFM!
Let's say my AFM requires anti-ice to be selected on 2 minutes before entering icing conditions - if I am intending to comply, then it would generally take me less than 2 minutes to climb AEO to your 1100' icing level, so I should be prepared to turn the system on just after lift-off or, more practically turn it on to start with.
What we generally do is calculate both figures and make an informed choice when we can see our take-off path.
If there is no time limitation, then generally my reduced take-off thrust with anti-ice OFF will provide me with adequate EGT/TGT margin to select the systems ON as required.
Our approach to the issue is simple: since it is more of an airmanship issue than anything else, we consider it to be part of the commander's operational discretion to ensure that the aircraft has the appropriate anti-ice protections in place before entering icing conditions. Moreover, our FOs will ensure that the commanders meet those requirements.
Stay Alive,