A study carried out in Norway and published in the journal Food and Chemical Toxicology in 2012 suggested establishing a Tolerable Daily Intake (TDI) for coumarin of 0.07mg per kg of bodyweight per day. The researchers commented that by sprinkling cinnamon on oatmeal porridge or drinking cinnamon-based tea regularly, adults and children can very easily exceed this amount.6
Based upon the conclusion of this study, if the average weight of an American male is 191 pounds (86.6kg), it could mean a maximum Tolerable Daily Intake of 6mg of coumarin. For an average American female (159 pounds or 72.1kg) it could mean a maximum of 5mg of coumarin per day.
In a document published in 2006, the German Federal Institute for Risk Assessment (BFR) suggested that 1kg of (cassia) cinnamon powder contains between 2.1 and 4.4g of coumarin7. If you estimate that powdered cassia cinnamon weighs approximately 0.56 g/cm3, a kilo of cassia cinnamon powder would equal 362.29 teaspoons. This suggests that a single teaspoon of cassia cinnamon powder could contain between 5.8 and 12.1mg of coumarin (which may be above the Tolerable Daily Intake for a smaller individual).