Bert,
Like Water said, time will depend on the cut of the brisket. Generally speaking, a flat will cook faster than a full packer. It's not about the weight, but rather, the thickness of the particular piece of meat.
Water's advice is spot on (as it always is). One thing that I'd add is to warn you about "the stall". The internal temp of your brisket will rise steadily after putting it in the smoker. Somewhere around 150 degrees or so, it might hit a wall and the temp will STOP climbing. The temp might even drop a little. You'll swear that something must be wrong, that your thermometer must be broken, or that you are probing the brisket incorrectly.
What is actually happening is that the brisket will be undergoing something called evaporative cooling. Basically, the brisket is sweating which cools it down. There's three ways to deal with the stall. First way is to shrug it off, open another beer and just give it time. The temp will start increasing again at some point. Second method is to wrap the brisket tightly in some foil and put it back on the smoker. Third option is to crank the temp up on your smoker (if you can) and power through the stall. One other thing to point out is that while the brisket is wrapped in foil you can put it your oven if you want/need to.
One downside of foiling is that it makes the bark of the brisket much softer.