Time means nothing. Its the internal temperature that matters. What was the temp when you pulled it.
You can cook a flat in 8-10 hours depending on its thickness and temp.
What kind of cooker are you using? Is this grate temp you speak of?
More info please!
Large Big Green Egg
Weber Kettle Blue 22
Weber Smokey Joe
Weber Gold Genesis (Sadly, this did not come with a long keyboard solo)
1 ~ 3-year old Princess
1 ~ 1-year old Caveman