It would be handy to know before the symptoms kicked in.
But 15 years? 
There's a good argument for not knowing when you are actually going to die as the countdown would be really hard to take however this condition is a gradual downward slope.
15 years before symptoms kick in would give you a lot of time to plan what you were going to do and in truth for a lot of people they would probably make the most of the next 15 years and live a far better quality of life than the rest of us who just stagger from day to day

I'd probably plan to budget my money so that I had a whale of a time for 15 years but then still had 5 years left, once I saw the symptoms coming then i'd be planning how to take back control my own way