I think both of you are misunderstanding each other.
When A50 is invoked you leave after 2 years, deal or no deal.
But as NSIS says it will take longer than 2 years to fully implement.
It can take an interminable amount of time. The negotiations will be long and tricky. The two sides are miles apart on fundamentals, such as free movement. We can't leave until the divorce settlement is reached. That can be any amount of time that all parties agree is necessary.
I'd be amazed if this is concluded in 2 years.
