That should be plenty of time to take it easy on the ride. I've done the entire BRP in two days. It makes for two somewhat long days, but it's certainly doable without breaking any laws. If you take my route, just know that the Fairfax County Parkway may have some traffic, but it shouldn't be that bad because by the time you get to it, it'll probably be around 10am and most of the rush hour is done.