How tall is too tall?

Seemingly there is a limit on how tall you can build your Lego tower before you are told to stop!
I took the 2 lads to the "BrickLive" event at the NEC in Birmingham.  A massive hall full of all things to do with Lego.  And lots of Lego to play with.
5* review from a 6 year old: "I'll give it a 100 thumbs up".
And "If there was a hotel there, I'd stay for 2 years".