I have played two games of true Mega Carcassonne (
this one and
this one), with all of the official expansions and mini-expansions avaiable at that time. Each game took about 9 hours though, so I wouldn't want to do that too often! The total tile count is somewhere in the 450-500 range.
I've done it two years in a row now with some friends from work and we are talking about doing it again this year so it can become a established as an annual tradition