Yes. For a 1st Offense DWI in Fairfax County, they follow the Virgina Mandatory rules of 5 days minimum in jail for a .15 BAC and 10 days jail for over .20 BAC on a DUI in Fairfax County.
If it is a 2nd offense, then there is even more mandatory jail time for DWI committed in Fairfax County and other Virginia jurisdictions.
Check out the Virginia DMV Drunk & Drugged Drivers Brochure for more information.