Yes guesswork.
The calculation here actually adds a few hours onto my estimate. Putting it at 9am for 4 pints of 568ml beer at 2.3 units to clear
How long does it take for alcohol to leave your system? Use this blood alcohol content calculator to find out how much over the drink-drive limit you are.
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But I'm sure meeting in the middle assuming they are playing it safe when their estimations. This is a car insurance company after all! Then perhaps 8am is probably a more reasonable answer. And at that point you are either clear altogether or very near to being clear and comfortably under the limit.
The tricky bit is for shift workers who do say a 6am start. They really have to be careful on knowing when to stop as they'll be on the road a fair bit earlier than your typical day shift worker.