Six weeks is the standard time they give. Mine told me that, too. Then, when he saw me at the first followup, which turned out to be three weeks, he cleared me to drive. It all depends upon how he assesses your progress. There may be some local laws involved, too -- I don't know.
I didn't drive right away, because the knee didn't feel quite right, and for the first week, I only drove locally (within 2 miles of the house). As the knee felt stronger, I expanded my range to PT (40 miles one way).
Your experience may be the same or different. I know my dad was cleared by his doctor to drive at about three weeks, too.