Hi there and welcome!
BKING and the others have provided some great info and I agree with em!
Briskets and pork butts are done when they are tender not by time or temp alone.
If your smoker runs at 275F you can estimate a little over 1hr a pound on that brisket. AGAIN the brisket is done when it is tender all over not based on the time estimate I provided at that smoker temp.
Confused yet hahahha?
I gave the time estimate so that if you want to eat at noon you will put that brisket in the night before.
With a brisket and a pork butt ALWAYS estimate how long it is generally gonna take (at 275F a little over an hours pound) then add 4 hours for safety. BTW briskets don't care what temp you cook them at so I always go 275F to save some time and nail my time estimates.
So if u want to eat at noon you will want the brisket to be done around 8am and give a 4 hour rest/"oh crap" buffer of time. If the brisket comes out done 4-5 hours early then that is perfect. You simply double wrap tightly in foil, wrap tightly in 3 bath towels and set it on the table/counter until just before noon (4 hrs later) where you will unwrap it and slice it while it is STILL piping hot! Trust me it will be piping hot 4 hours later if done this way and the rest time will do the meat wonders!
If the brisket doesnt finish 4 hours early well you have 4 hours of time for it to still finish.
Finally I always recommend people trim their brisket where they cut away the thin portion of the flat to where what is left behind of the flat is roughly the same thickness all over. The really thin portion will just burn up and be useless so repurposing that good meat is a way better alternative.
I always show this diagram to help visualize what I am saying:
View attachment 440405
You can
see my trimming post here that shows burnt up thin portions of brisket flat and a good trimmed brisket flat.
The lack of poor estimation is probably the #1 reason why people have failures on their briskets... especially with first attempts so please head the warnings with this info!
Estimate properly according to your smoker temp. Trim it well.
Put a thermometer in the center most thickest part of the flat and when it is around 200F start checking for tenderness and pull when it is tender (I stab all over with a bamboo kabob skewer to check).
Do these things and you should have a pretty good 1st brisket attempt that you can learn from and improve from. After a few attempts you will nail it im sure! :)