There are two answers to this question.
One is how thick is the base layer going to be?
The other is how thick is the asphalt surface going to be?
All blacktop driveways with a thirty year life expectancy should have a minimum compacted base layer of 4 inches. In some cases more than 4 inches are required depending on what type of soil is underneath. Once the old driveway is tore out an experienced professional will identify soft spots in the driveway that will need to be dug up and the base layer in these spots will be at a depth 6.5 inches.
If an asphalt driveway has a poorly laid base common problems that you might see are settling in parking spots, and ruts from usage.
At Ace blacktop we give the customer 24 hours to view the base layer of their driveway before the asphalt surface is installed. At this time the consumer can check the depth, grade, and drainage to ensure satisfaction. This process can be expedited by request. The asphalt surface of your driveway should be laid at 3 ¼ inches then compacted to 2 ½ inches. A thicker asphalt surface may be needed depending on what type of traffic your driveway sees. As a general rule the heavier traffic the thicker the surface.
If too thin of a surface layer is used your asphalt driveway will be susceptible to premature aging. Traffic, sun, and frost are the three main components that will age your driveway. Signs of an aged driveway include premature cracking, a worn rough texture, and a dry and brittle surface.
Your best bet is a 4 inch compacted base with 2 ½ compacted asphalt surface. This will produce a beautiful driveway with a 30 year life expectancy if cared for properly.