I know that on my carpenter i've had up to 280 boxes in overflow. And i experienced a bug where i had gotten rid of all the boxes in my actualy slots but still had 260 or so in overflow. And the game was no longer registering me as overburdened.
and yes i was at about 29,000/110
*edit* i made a large batch of t3 boxes last night... by time i was done i had well over 500 boxes in overflow