I this "lean" is measured at the top, and the buildings weight is equally distributed from top to bottom (i.e. its centre of gravity is right in the middle) it

I this "lean" is measured at the top, and the buildings weight is equally distributed from top to bottom (i.e. its centre of gravity is right in the middle) it can lean half its radius until the CoG is no longer above its support and it should topple. That's very rough of course. Many bad things could happen much earlier, such as unexpected forces causing structural damage. Note also that the WTC didn't exactly "fall over". It collapsed on itself.