Keep in mind that labor guides are just that; a guide only. Guides such as Chiltons, Motor, and so on usually have something in the foreword about this and that additional time should be allowed for rust, frozen or mangled fasteners, or even cleaning if the situation warrants it.
If the labor guides says 1 hour to change spark plugs and it turns out to be a 3 hour job because the plugs are frozen in place, threads are pulling when the plugs are removed, and so on then the labor guide has to go out the window.