So 13 lbs, once trimmed, may take up to 18 hours easy.

I had a 14 lb brisket that I cooked yesterday and I had to pull it a tad early so I could get the burnt ends done and let it sit long enough. It cooked for just short of 18 hours at 225.

The trick isn't time, it's doneness. Some people aim for certain degrees like 200-205, but it's done once you can insert the temp probe and it goes in and out like it's going into butter.

And as other people said, it really needs to rest for 1-4 hours after being pulled. If you have an extra cooler, wrap it in foil, put in the cooler, and cover with towels (I refer to these kindly as "meat towels" since they retain that smokey flavorful smell). You will take it out 3 hours later and it will still be too hot to handle and all the juices settle.