I think its very difficult to answer that question as it depends on the kava, the mix, how much you ate for lunch, your hydration level etc.
I usually have a level of kava feeling when I know its time to go home - and continue drinking at home if needed. But sometimes you get caught out - like on my last trip to Vanuatu had just 4-5 shells of borogu from Lini's nakamal 15km from town. My head felt fine but as soon as I stood up my legs went and double vision kicked in quickly - out of the blue. Had to have a 30 minute nap before driving back, and even then it was a white-knuckle drive at 20mph..
Personally I don't like driving after any kava - even just on my motorbike to the shop 500m up the road.