A study of 46 adults aged 22–49 years and of mainly European descent found an average brain volume of 1273.6 cm3 for men, ranging from 1052.9 to 1498.5 cm3, and 1131.1 cm3 for women, ranging from 974.9 to 1398.1 cm3.  From wikipedia. Let's average it out and say 1,200 cm3. Poop is about 75% water, so we'll go with that. The density of water is about 1 g/cm3. So, the average human skull can be clogged with 1,200g of shit. A Human of 160pounds poops about 1 pound a day or about .45kg. It would take about 3 days to clog the average skull.