I've never checked. As soon as I see the temp start to rise and smoke is produced I am off to take care of chores, watch a ball game on TV or something productive. I don't go back till the smoke is within 30 minutes of whatever time I selected. If cooking to temp, I don't check till the next day.
Small edit: How long it takes to get to temp can depend upon the ambient temperature inside the box as well as the meat load. But as I said, I really never checked.