About a year, she met him when he was 17,and he died aged 18 3 months and 3 weeks
That can't be right. He died at 17 - Isaac and he discuss this during his prefuneral. (Book, not movie at least.)
Sorry, I had the same question and google led me here.
The only reference to dates I can find in the book are that they arrive in Amsterdam May 4th, and they visit his grave on Bastille day, July 14th. If you do some math around her "X Days A.G." reference(s), he died July 1st. So, assuming Amsterdam is where they officially became a couple, two months. Total time knowing each other, I do not know. The book opens with "Late in the winter of my 17th year", which could be literal or figurative.
Best Estimate? They knew each other around 4 - 6 months.