BlueDC is correct. The HOV hours on I-66 inside the Beltway are 06:30 to 09:00 eastbound and 16:00 to 18:30 westbound. As mentioned, things are bad at the limits. People line up on the shoulder to drive in to DC after 09:00. However, once you have cleared I-66 then the toll road can get ugly. Bring you E-Z Pass if you have one.

Also, if you drive in late there are a lot of eastbound commuters and people going into DC so it can get backed up on the toll road spur to I-66.

Consider getting a hotel in Tysons, which is a bit closer. Off-peak drive time is < 15 min from there and there are some useful back roads and other options to get into DC. In any case it eliminates the potential toll road tie up from the equation. From Tysons you only need to worry about I-66. In Reston you also have to deal with the traffic pattern into Tysons and toll booth tie ups.

You might get a good deal PL in Rosslyn or Arlington. It would be a bit crappy to stay out in Virginia suburbia. It really sucks and would spoil your vacation.