I would like to know what the actual amount of time is for a trained mechanic to change the spark plugs in a 2003, ford, ranger xlt, 4.0 l, 6 cyl., 4 WD, automatic ? The local Ford service/dealer garage tells me it is a 2 hour job. Keep in mind this is just to replace the plugs themselves only. My experience tells me it is actually closer to a 30 minute job. What is the real answer ?
The two hour quote is most likely the book hours it takes to change the spark plugs. Most shops here in the States use a book hours estimation on what it takes to do the work. This is figured (I believe) on what the time it takes the average mechanic to do the work. For your vehicle and the job being done, the shop is estimating two hours based off of these figures. This is done in an attempt to make the playing field level for all shops. If you get a book hour estimate, it may not take the mechanic two hours to get the job done, but you are going to get charged that amount at whatever the book hour rate of the shop you are taking your vehicle to. This is more or less an honest estimate of what it's going to take. If it actually takes longer, you will not pay any more for the service but the two hours.
Two hours in and of itself for the job seems reasonable for "book hours", mainly because this also includes things like getting the vehicle into the shop, collecting the parts and materials needed to do the work, testing the vehicle after work is done, inspection of the vehicle to see if there's any other "stuff" which should get done, etc. It's just considered a fair estimate of the work which is needed. It's fair to the shop so they get paid for their time. It's fair for you because you know going in what it's going to cost.
This is a bit tricky to answer because it is basically variable. Generally, shops will bill a full hour for every minute into each new hour. So that would be why they'd estimate 2 hours. They are 99.99% sure that it will never take longer than 2 hours, but it could take them upwards of 61 minutes.
For starters, it could be done in 30 minutes, but it could easily take longer. Even a "trained mechanic" will not have worked on every vehicle design (which can add time to the job), and even if they have, they haven't worked on your vehicle necessarily. So they may have to spend some time figuring out how to finagle their way into certain spaces. Also, some designs require the removal of different components to access all the spark plugs.
I don't know precisely about the design of your Ranger, but I could easily see it taking over an hour (would probably take me about 70-90 minutes, start-to-finish, as a guess) which would be billed as 2 hours.
This is the reason that I personally would do that job myself. If you have the ability, skill, and time, then I'd say do it yourself as long as you know what you're doing.
All that said, anything relating to cost/shopping is a bit outside the scope of this stack. So all I can say is that I can see it taking over an hour, and I can see if I had worked on that vehicle before (especially several times) it would probably take closer to 30 minutes. It's just a matter of familiarization with the vehicle.