Asphalt driveway thickness depends on the soil that's being paved on and the industry standards of the region you live in.
What is the soil like in the region you live in?
Here in Denver Colorado, we have two types of soils we pave on. The first, on the front range, the soils tend to be expansive soils so most contractors excavate and install 3" to 6" of road base and then install 3" of compacted hot asphalt. All of this work is done in a 1 to 2 day period. In the mountains just west of Denver, however, the soil is decomposed granite which is almost identical to that of a structural road base. You couldn’t ask for a better sub-grade so most contractors don't install road base and pave 3" of compacted hot asphalt. Usually, this can be done in one mobilization and in the same day.
In other parts of the country, like Kentucky for example, the standard practice is to excavate and install 6" of base, wait 2 to 3 weeks and then return and install 2" of compacted hot asphalt.
Now usually, a residential driveway has nothing more than everyday passenger vehicles driving on them, but if you have delivery trucks, trash collecting trucks or any other large commercial vehicles that will be driving on your driveway on a regular basis, make sure you let the contractor you plan on hiring know that important information. If it's at all possible, you really should try and eliminate the need for these vehicles driving on your asphalt driveway. These heavy vehicles can cause severe damage very fast or over time.
Now, the only way to ensure the right thickness of base and asphalt for your soil type, is to hire a soils engineer to evaluate your soil and come up with a pavement design for you. However, that can multiply the price of an asphalt driveway by three times! Even still, there's no guarantee that it wont crack. The reputable contractors in your area will know what the proper thickness the asphalt should be for your driveway. And as the saying goes... your asphalt driveway is only as good as the sub-grade beneath it.
Paving a driveway too thin is actually a common rip off practice by the "fly by night" unethical contractors! Watch the video below about the # most common asphalt scam!