It depends on the distance, or how it was expressed. Less than 12 hours, or exact times, probably means just the actual driving time before any stops. Really long spans of time, or when someone says something fuzzy like "a half-day drive," you can probably assume are accounting for a break or two.

To answer your specific question, if someone said "a 10 hour drive" to me, I would assume they meant 10 actual road hours, before breaks. I would expect them to actually arrive in an 11 to 12 hour time frame.