btcg gave you a good guidelines, the cooker and temperature makes a difference, as would the meat itself. A small flat like that is probably better done at a lower temperature, it gives you a little more room to not pay attention to when it is done. My experience has been that a 4 to 5 lb flat done hot-n-fast will rise very quickly out of the stall.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."