Smokingal,
One and a half hours per pound is really more of a guideline. When it comes to smoking, every piece of meat is different. You have to go by the internal temperature of the meat.
For brisket (and holy cow! 17 pounds is huge! But go big or go home, I always say) you want to go to an internal temp of 190°-195° for slicing, 200°-205° for pulling. Many people smoke until it hits 160°-165°, then wrap it tightly in foil and continue w/o smoke until it hits the desired temperature.
Be aware that when it gets somewhere between 140° and 155°, it will hit the "stall", and might sit at that temperature for hours. Sometimes they even go down a degree or two. But Oldschoolbbq is right, don't try to rush it along. It's at that point when the tough collagens in the connective tissues are converting into softer gelatins. How long it takes depends on how much it has to convert. Just be extra patient, and let it do its thing.
After it hits the desired temp, take it off the smoker, wrap it in a large towel, and set the whole thing into an empty ice chest (or an off oven, in a pinch) and let it rest for at least an hour.
The biggest brisket I ever did was a 15 pounder from Costco. I think it took 26 hours. So just roughly gauge your start time to when you want it to finish. Keep in mind that it's okay to finish ahead of schedule. It will stay warm in the chest for hours.
Hope this helps.