How do auto repairshops determine the number of shop hours required for a specific repair?

I know there is a reference guide which states the number of hours each repair should take. Repair estimates are based on the prescribed number of hours X shop hourly rate. Do you know the name of this reference or where to look up the same information online?
I ask this question because I’d like a source from which to determine how time consuming certain repairs are and whether we can consider tackling them at home. I have a 2002 caravan that needs an alternator. . . just had the other car in for over $2000 in repairs, the repair budget is wiped out.

I had similar problems with my Honda Prelude and last month. Get a free auto warranty quote and see if you can get the policy before you need to get your car fixed. http://www.simpleautoquotes.com/Auto-Warranty.html

Automotive Mechanical Estimating How To make repair estimates