I'd say no more than a tea spoon. I cleaned it out and have since put on another 1500kms and its still clean in there.
I disconnected the battery and left it overnight, thinking that it would reset the computer and well, who knows… filled up and reset all the dashboard values and wouldn't you know it today I got 350kms & 370kms out of the next two fill ups!
Mileage is back up and even better than before!!