It could take a couple days to thaw completely. I believe the rule of thumb is
1.5 hours per pound at 250 degrees. Each piece of meat is different and you'll know when it's done when you can stick your temp probe in the thickest part of the flat and it goes in like you're sticking it into butter. Use temp as a guide but let the meat tell you when it's done. I would start looking to see if it's done when it hits 185ish.
Cooking time can vary depending on your technique. If you use the Texas Crutch or foiling the meat when you hit "The Stall," which comes around 150F, you'll cook it faster. If not, you're going to be stuck at 150 for quite a while (hours) before your hunk of meat continues up beyond 150 to your final IT.