It may not have been stated in front of the jury, but was in his notes. As Jane said, there were trace amounts in her system, suggesting she hadn't been a user for awhile.
Depending on how she took the drug and how frequent would determine the length of time it would be detectable in her body.
So a single use 3 days in urine or 48 hours in blood (approx) for example.
For a heavy user 30 days in urine or 25 days in blood.
I don't know if Sheila was a heavy user or not? so if cannabis was detected she could have been using the drug quite recently?