That is *possible*, if your vehicle is well-built, you are a good driver, and everything goes well (no traffic, no inclement weather, no breakage). If you have never run the trail before (perhaps why you are asking), I would bet against it, tho, and any time to check out Little Sluice or Rubicon Springs is gonna hash your schedule.
If you elect to day-trip, consider going in Loon to Ellis Creek. If that takes you five hours, then either exit back at Loon (SW) or head due West and exit at Wentworth Springs. If you've spent less than three hours to get to Ellis Creek, consider continuing into Little Sluice and exiting at WS or Loon that evening. If you're at Ellis inside of an hour, then consider heading all the way through.
IMHO, making a day trip of the Rubicon is a bit of a waste, as you're driving all the time, and it is hard to take time and enjoy the beauty of the resources around you. Sure, you can claim that you've done it, but a trip on the Rubicon is less incredible without breaks to enjoy a cold soda overlooking Spider, a short hike uphill of Buck to Rockbound, a quick spash at Rubicon Springs waterfall, and a long smoke overlooking the Rubicon River from Observation Point. Take an extra day off and smell the roses...
Randii