At the very minimum a thickness of 4″ is needed to achieve strength and long-term integrity. This goes with most flatwork in residential settings.
The thickness gets bumped up to 6″ for the approach of the driveway. That’s the entry from the curbside of the road across the sidewalk and abuts to the driveway.
If your location has heavy traffic such as a weekly garbage disposal truck (heavy), consider adding an inch to the thickness for an increase in strength.
Another option is metal reinforcement such as rebar or 6×6 welded wire.