Should be same as auto industry. Book time is a guide, or estimate. In the auto industry, we dont charge over book time unless we can clearly indicate a reason why(broken bolts, severe damage, ect..)
Also, book time sometimes doesnt allow for combined operations. As an example, You estimate a engine R&R & 10 hours. You also have a front pump seal leaking on the transmission. Book time on that is 6 hours. However, since you already have the engine out, and access to the front seal on the trans, you are SUPPOSED to adjust your labor time on the engine R&R and just add time to it to do the seal. Sometimes mechanics/advisors will get greedy and try and bill both the Engine R&R labor of 10 hours, AND the front pump seal labor of 6 hours. Seen it happen many times and it makes my skin crawl. Should have been for like 11 hours total max.
All you can do is confront them and ask about it, make them explain. If you are being billed 24 hours of labor and they only had it for a 8 hour work day........ thats a problem and common sense will be on your side. Granted, mechanics can beat book time if they are good, but they are not that fast.
In most places, billing for something you didnt do is fraud. This applies to labor as well.