The manufacturer will have standard repair times but I'll be the first to tell you they're bull####.
Most manufacturers get their times by having a mechanic perform the repair 3 times back to back on a brand new and bone stock unit with all tools and specialty equipment at arm's reach. They use the fastest of the 3 times as the standard. In reality count on it taking 30-50% longer unless the guy doing it does that job 2 or 3 times a week, every week.
but is there a way for me the customer. To find out what the "book time" is BEFORE putting truck in shop?
Also my experiences have been the opposite of yours. For instance last week the dealer wanted to charge me 3 hours book time to change a sensor. I changed it myself with hand tools, outside in 20 degree weather, in less than an hour.
So how long would it have taken a tech inside a warm shop?