Answer: There are several factors that will determine how thick the gravel should be on a gravel driveway and the intended use of it.
Many homeowners build a gravel driveway as a temporary surface until a hard surface driveway is installed such as asphalt, concrete or pavers. In this case, most asphalt, concrete and paver contractors would prefer to to the job thoroughly from the bottom up in order to warranty their work. So investing too much money into a temporary gravel driveway just isn't worth the cost. In this case, 3" to 4" of gravel will suffice for 1 or 1 years.
On the other hand, many homeowners want to build a gravel driveway as a permanent structure with no plans to install a hard surface driveway. In this case, a sold base material like recycled concrete, limestone or road base should be installed 4" to 6" thick and compacted well. The variation in thickness is determined by the use of the driveway.
For common vehicles, 4" should suffice. For heavy vehicles like RV's, boats and coaches as well as trucks like trash trucks, propane trucks and septic trucks, 6" would better suffice. This is all dependent on the soil in your area.
Once the base is installed, then 3" to 4" of gravel, crushed stone or recycled concrete can be added to the top to add visual interest to the driveway.