If your briskets are all different sizes then it will take different times depending on the size, weight, stubborness of the meat, etc.
The best thing you can do for yourself is to try and purchase briskets that are very similar in weight and size.
This will allow you to estimate a little better.Ultimately, temperature is the indicator on when meat is finished
and I highly recommend a good digital probe meat thermometer
. I usually cook mine to around 185 however some take it on up close to 200 for that fall-apart effect.
Having done my share of briskets, I have noticed that time closely relates to when a certain temperature will be attained to some degree anyway(pun intended) ;-)
To get it down to a science, you want to allow around 1.6 hours per pound and then give yourself a few hours as a buffer. This figures out to be about 96 Minutes per pound and seems to be more accurate than the 1.5 hour/lb that we generally talk about.
It is easy to keep briskets hot for several hours once they are done.. it is not easy to keep people happy when they are waiting to eat so give yourself more time than you think you need and you will be glad you did later.
When you are smoking meat of different sizes, you have to look at each piece of meat separately to figure when it wil be done and this may mean using more than one thermometer to keep from poking and creating leak holes in the meat during the cooking process.
If you are not in charge of buying the meat and end up with various sizes, you can sort the briskets into 3 or 4 groups of similar sizes and put them into the smoker in intervals beginning with the largest group.
Allow yourself at least a 2 hour buffer for each group to be on the safe side.
It is no different when cooking different types of meat..
If you are cooking chicken, ribs and brisket and you want them all to get done at the same time then you would figure out your cook times and subtract the cook time from your finish goal time to know when to insert the meat into the smoker.
In the example above, If I wanted to have the meat ready by 2:00 PM on Saturday, I would place my 10 pound brisket in at 8:00 PM on Friday night which gives me 16 hours based on 1.6 hours/lb + a 2 hour buffer, the ribs would go in at 6:00 AM on Saturday morning which gives me a good 6 hours + 2 hours wrapped in foil and in the igloo cooler for further tenderizing. The chickens would go in promptly at around 9:30 AM on Saturday to allow about 4 hours + some good resting time.
The formula is S=F-C:
F= Finish Time
S= Start Time
C= Cook Time
Hope this helps!!