It can take anywhere from 25 minutes to an hour and a half, depending on traffic. If you avoid the rush hour, 45 minutes is a good bet from the Sheraton to most parts of northwest/downtown DC. Just follow the Toll Road to I-66 east and then the Roosevelt or Key Bridge into the city.

I-66 almost always jams eastbound at the Toll Road merge, but it's usually only a minor delay that clears up within half a mile.

If it's just going to be you in the car---no passengers---you can't legally drive into DC this way between 6:30 and 9:30 a.m. and out of DC between 3:30 and 6:30 p.m. I-66 is HOV-2 only inside the Beltway during these hours.

Keep in mind that parking is pretty expensive in DC on weekdays, so you might want to drive to the West or East Falls Church Metro station and take the train in. Parking at these stations fills up early, though (probably before 7:30 a.m.) so you may be out of luck if you try to go in any later. There's some free street parking up the hill from the Rosslyn Metro station, the first stop across the river from DC and convenient to I-66. If you'd like, I can post directions to the Rosslyn-area parking here on this thread.