Seems to be around <10mg.

https://today.uic.edu/low-dose-thc-can-relieve-stress-more-does-just-the-opposite/ > The low-dose group received a capsule containing 7.5 milligrams of THC; the moderate-dose group received a capsule containing 12.5 milligrams of THC; and a placebo group received a capsule containing none. Neither the participants nor the researchers knew who was in each group.

> “The doses used in the study produce effects that are equivalent to only a few puffs of a cannabis cigarette,” said Childs, noting that it is difficult to compare doses of smoked cannabis to doses of ingested THC.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3018238/ (very low dose)

> Healthy volunteers (n=11) received capsules containing placebo or THC (2.5mg)

https://pubmed.ncbi.nlm.nih.gov/16177808/ > We examined the effects of a single sublingual 5 mg dose of THC