From Thomas Jefferson Encyclopedia
Archibald Stuart(1757-1832), lawyer and judge, read law under Thomas Jefferson. The relations between Stuart, although years younger, and Jefferson were friendly. He attended William and Mary from 1777-1780. In March 1781, Stuart served in the Augusta and Rockbridge militia at the battle of Guilford Courthouse, North Carolina.
Family tradition that Stuart read law at Monticello is based on this brief notation from a biographical sketch of Archibald Stuart by his son, Alexander Hugh Holmes Stuart:
"...Archibald Stuart spent the greater part of next year (?) in the study of law with Mr. Jefferson" 
Since Stuart was a student and soldier up until 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 that Jefferson's interest in it and young students never wavered. After Stuart become a lawyer, he 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.