Maybe closer to 1/3 than 1/4 but probably not as much as 1/2.
My starting guesstimate for smoking brisket is about 1.5 hours per pound at 200-225oF but it could be anywhere from 1 to 2+ hours - all depends on the cow. If you're using Bradley pucks, most folks find that 4-5 hours of smoke (12-15 bisquettes) is enough (especially if you're using mesquite which is pretty strong). After that, just keep the temp even and let it go till it's ready. Some people (including me on occasion) transfer to the oven at that point (temp easier to maintain). A lot of people, including many "guru level" pitmasters and competitors (neither description even remotely describes my limited skills) foil or wrap. I don't so you'll have to check elsewhere for how that works. I do use a pan of water to maintain a moist environment in both the smoker and (if used) the oven and I do FTC before serving or storing.
If you have a small packer (10-12 pounds) you're going to plan for a 15-18 hour cook so 18-22 hours sounds like a reasonable guess for 2 or 3 of them. Remember (seems I can't say this enough) time and internal temperature DON'T tell you when a brisket is done; only the brisket tells you. If you're smoking 3 identical looking 12-pounders in the same pit, one could take 10 hours, another 18, and the last 22+.
Which all brings up the point of timing your cook. Many of us find it best to smoke brisket (and pork butt) well ahead of the serving date, store it in the fridge or freezer, and reheat it (gently) when needed. I'm making brisket and ribs for a party at my house later this month and will probably smoke the brisket 2-3 days before the event. Many people will tell you the flavor is even better after storage (and they're pretty much right) and not having the pressure of trying to have a major part of your meal ready at exactly the right time is removed.
I'm not doing it this time but I usually buy two briskets at a time and smoke one flat and both points together. I serve the flat (sliced) and vacuum pack and freeze the points for later use (burnt ends, chili, sandwiches, etc.). Oh yeah, the other flat easily makes corned beef so much better than that crap you buy in the plastic bags that you'll never look at them again.
I'm tired of trying to think now and it's cocktail time. Aloha.