From Thomas Jefferson Encyclopedia
The relations between Stuart, although years younger, and Jefferson were friendly. Family tradition that Stuart read law at Monticello is based on thie brief notation from a biographical sketch of Archibald Stuary by his son, Alexander Hugh Holmes Stuart:
"...Archibald stuart spent the greater part of next year (?) in the study of law with Mr. Jefferson"  Stuart becoming a lawyer looked after Jefferson's interests on "the other side of the mountain", which included collection debts owed for the sale of Monticello nails and paying taxes on the Natural Bridge.
Since Stuart was a student at William and Mary College about 1781 it can be assumed that he was at Monticello after this date. This period was well beyond August, 1774 when jefferson ceased to practice law' however, it must be noted his interest in it and young students never wavered.
Excerpt from James A. Bear, Jr., "Card Tables", October 6, 1962,