It takes about 45 minutes to reach a postabsorbtive state, where BAC maxes out and the alcohol is evenly distributed in the body. Many states have provisions for DUI even though the BAC is lower than legal thresholds.
A lot depends on the driver's attitude toward the police,
I know a guy who blew a .02 and got a DWI. He couldn’t pass the sobriety tests because he was hammered on Benedryl.