You can’t farm with electricity.
I say down to 90% in 2 years, because people would eat what they could find for quite a while, but I would concede that most of that 90% would be gone in 6-8 months.
Of course, there is no logical reason why oil would completely disappear overnight. Natural gas and coal are also fossil fuels, and we have plenty of each.
In an emergency, cars and farm vehicles that run on natural gas could be developed and produced quickly.