Howdy,Audrey.
You didn't say what you are cookin' on,so I'll guess a CS.
Smokin' Okie has done a great job with his Brisket 101 and there are many good threads on the search function.
He's probably still runnin' amok,tryin' to eat Austin.
A lot will depend on the grade ,interspersed fat,and what kinda animal.
I'm thinking you are cooking down around 200�-225�.
I hope you have an internal probe thermometer,'cause when that rascal gets around 180-185�� you could be gettin' close.
It would be easy to say around 1-1.25 hrs./lb.,but little flats will lie to ya.
If you are not using probes ,you need to start checking it at about 3 hrs. for tender.
Smokin' will chime in and tell ya "its done when its done".
Try it this way first,take good notes and save the tricks for when this doesn't work.