First off, its called concrete! Cement is just an ingredient in concrete. Sorry, just a pet peeve of mine as a construction engineer.
Although your driveway will prob be hard enough to walk on the same day, I would give it several days to cure before driving/parking on it depending on the vehicle. Most importantly, talk with the supplier; he should be able to tell you for sure. Hopefully you didn't get leftovers from another job and the concrete was cured correctly. It will take 28 days on average to reach its design strength (prob 3000psi) and then the hardening drops off although it will get harder theoretically for all time. Hope this helps somewhat! Good luck!