As has been asked, are you new to riding dirt or just dville? If you're just new to downieville, it shouldn't take you more than four hours. Maybe about 4 tops if you stop to eat and take pics and dick about.
Definitely visit the good folks at Yuba. They'll hook you up with trail maps and they've got sweet rides to rent, too.
The signage on the trails has improved over the years and you shouldn't really have too much trouble finding your way down the hill. Remember that since they run shuttles on a regular basis, there are bound to be more folks coming along every so often. As long as you don't get too far off of the beaten path... As has been suggested, you might also try following a group that knows the area. Though that could be a recipe for disaster if they're really rippin'.
I'd definitely recommend their shuttle service, too. As has been mentioned above, you *could* drive your car to the top but if don't have a second car to use to fetch it, riding back to the top would suck in a rather large way.
Be sure to pack tubes, tools, food, and water. And don't forget that, even though it's June, you are riding in the Sierras.
Have a great time!
