Greetings NKyGuy,
Peace be with you.
I have not heard "when" you should tell her, but I did hear this which I may share with you. If you honestly feel that you cannot go to the Convention and you truly wish not to attend, then at a time you should come to your wife and tell her what is "true" in your heart as to how you feel. Let her know that you do not wish to attend, remind her of how you have problems with the teachings, and that you still love her and do not wish for there to be an ending of the marriage because of this. Always keep reminding her that you love her and still care for her, but you do not wish to attend the Convention because of how you feel. She may take it well, she may not. Many times if they see you still love them and that you are still a kind person, they may not bring great persecution upon you. I do not know what will transpire, but if you know you cannot attend it is in your best interest that you do not attend. If you do this out of love and let love direct you, things will go for her benefit as well as yours.. spiritually. Rely on the Christ and let his Spirit direct you.
Take care dear brother and have the GREATEST of peace.
Aaron